Population density refers to the number of people who live in a defined land area (usually square miles or square kilometers). So, if two million people live in ten square miles, the population density is two hundred thousand people per square mile. Population density is usually highest in urban areas, specifically cities like New York City, which keep growing but have a maximum land area. Los Angeles has a similar population level to New York City, but much more room for that population to inhabit. Of course, Los Angeles would still have a much higher population density than, say, the Rocky Mountains or the Alaskan wilderness.

More than half of the world’s seven billion people live in Asia. The next highest continent is Africa, with a little over a billion people. Approximately seven hundred million people live in Europe; four hundred million in South America; five hundred million in North America.

A “primate city” is the largest city in a region. In order to be a “primate city” it must be disproportionately bigger than other cities in the region, primarily because those people who live in the surrounding area migrate to the “primate city” in search of work, education, or entertainment.

Oceania (which includes Australia, New Zealand, and many of the surrounding Pacific Islands) is by far the least heavily populated continent on Earth—excluding, obviously, Antarctica. Oceania has less than fifty million people living there. The next lowest—South America—has a population of four hundred million people.

Apart from Sub-Saharan Africa, all of the listed regions are considered to be some of the most densely populated on Earth. The Indian subcontinent (which generally includes Bangladesh), East Asia (which includes Eastern China and Japan), Western Europe (which includes the Netherlands and the United Kingdom), and the East Coast of the United States (which includes New York City, Philadelphia, Miami, and Washington D.C.) are the four regions of the world that are the most densely populated. South-Eastern Brazil and South-East Asia could also be added to this list.

The vast majority of population growth is taking place in the developing world. In recent years, more than ninety percent of population growth can be accounted for by countries in the developing world. These countries continue to grow to represent a larger and larger share of the world’s population.

China and India are the two most populous nations on Earth; each with over a billion people. China is currently the most populous nation in the world, but most experts agree that India will surpass China sometime in the next couple of decades due to India’s much faster rate of population growth.

Jakarta, Indonesia is the world's second most populous city, with a population of close to . New York city has a population of over . Shanghai has just a slightly larger population than New York. Beijing has an estimated population of about . Delhi has a population of just under . All of these cities are continuing to grow rapidly in size.

Since the question speaks specifically of farmers in relation to the viable or arable land, we can rule out the answers dealing with physiological density, which deals with people in general in relation to the land. Agricultural density measures the number of farmers per unit area of farmland. If there is more land than farmers, the density of farmers in that land will be low, and thus more sustainable and efficient. If there are more farmers than land, land will be scarce and the density of farmers working on it will be high, and agricultural production will be inefficient. Thus this question of a small amount of farmers on a large amount of land is measured as a low agricultural density.

A decrease in arable land has the potential to affect all four areas listed. However, only physiological density (people per unit of area of arable land) is directly tied to arable land area.