Descartes is an Englightenment-era philosopher, scientist, and mathematician, and his work in all three fields continues to be hugely influential today. He is famous for his reductive inquiry into the nature of existence that led him to one simple truth: "I think, therefore I am." Essentially, by doubting his existence, he was proving his existence, for for there to be doubt there must be something to do the doubting.

Montesquieu is a famous Enlightenment philosopher who wrote in his seminal work The Spirit of Laws that, to protect against tyranny, a democratic government should be divided into separate branches: an executive branch to enforce the laws, a legislative branch to pass the laws, and a judicial branch to challenge the laws.

Enlightened despotism (also called enlightened absolutism) was a form of absolute monarchy that existed in the eighteenth and early nineteenth centuries. The rulers, or enlightened despots, were heavily influenced by the ideas and theories spread by the Enlightenment thinkers of the time, particularly ideas concerning rationalism, freedom, and sponsorship of the arts and sciences. Of the European leaders listed in the answer choices, only Catherine the Great was considered an enlightened despot.

Mendeleev is most famous for his work in "perfecting" the periodic table of elements. His work allowed people to understand similarities and differences between chemical elements with greater clarity. The theory of relativity was an idea of Albert Einstein's, and Newton invented the system of calculus. Alchemy, the practice of trying to turn materials into precious metals, was very popular during the Scientific Revolution.

Although slightly less revered in popular history, the Northern Renaissance was no less significant or accomplished than the Italian Renaissance. Albrecht Durer is probably the most famous of the Northern Renaissance artists. He is renowned for his woodcuts and his watercolors, as well as his theoretical writings on mathematics and perspective. 

Isaac Newton and Gottfried Leibniz are two prominent Enlightenment Era scientists and mathematicians. Seemingly they both independently devised a system of calculus that allowed them to make calculations on things previously impossible. It is generally accepted that Newton invented calculus first, but as he did not publish this information for decades scholars have to rely on clues in his publications and on Newton's word (notoriously unreliable). The two men battled for acclaim for the invention during their lifetimes, but it is Newton who is most commonly remembered as the inventor of calculus (at least in the English speaking world), despite the fact that Leibniz was the first to publish on the matter. Calculus was vital to some of the scientific breakthroughs of Newton's lifetime and has been continued to be ever since. 

The geocentric model of the universe was originally developed by many ancient civilizations, and popularized by the Greek philosopher and scientist, Aristotle. It was widely accepted by scientific-minded individuals for over a thousand years. In the geocentric model, the Earth is at the center of the universe, and everything else orbits around it; however, in the sixteenth century, the Polish astronomer, Nicholas Copernicus, developed his theory that the sun was in fact the center of the universe. This event is called the Copernican Revolution, and his model was called the heliocentric model of the universe.

Mary Wollstonecraft wrote A Vindication on the Rights of Women, in 1792. In the text she argues that women are not naturally inferior or less intelligent than men, but merely appear so because they were no allowed an education and were forced to be subservient to men. She imagines a society founded on reason and equality, in which women are given every right and opportunity that men are given. 

Voltaire was an Enlightenment-Era philosopher who was most famous for his vehement attacks on the established religious orders—particularly the Catholic Church. He wrote passionately, advocating for freedom of religion and the separation of church and state. This naturally caused Voltaire to be condemned widely by the powers that be (although he was a good friend of Catherine the Great), and he had many causes to fear for his life, but he never stopped his polemical writings. His most famous work, Candide, is a scathing critique of the established order of the European world.

A Treatise of Human Nature and An Enquiry Concerning Human Understanding are two of the most famous works of the Scottish Enlightenment philosopher David Hume. Whilst you might not have recognized the works as immediately belonging to Hume, their titles ought to have been a clue. Hume was primarily concerned with understanding the relationship between human nature and human understanding. He was a particularly prominent supporter of empiricism and moral relativism.