"Prone" and "apt" both mean liable or likely. "Fetid" means foul or rancid. "Futile" means hopeless or pointless. "Ambivalent" means conflicting or contradictory. "Axiomatic" means understood or accepted.

"Ironclad" and "abiding" both mean fixed or rigid. "Colloquial" means particular to a certain area. "Percipient" means alert or awake. "Furtive" means sneaky or secretive. "Quaint" means oddly nostalgic. 

"Weary" means tired or exhausted. "Outrageous" means shockingly bad. "Elegant" means graceful and stylish. "Talkative" means talking too much or chatty. "Amiable" means friendly and pleasant.

"Horrid" means dreadful or causing horror, so "dreadful" is the correct answer. None of the other answer choices are close in meaning to "horrid": "immaculate" means extremey neat and clean; "eternal" means lasting forever; "credible" means believable; and "shrewd" means sharp-witted or astute.

When an object is "opaque," it does not allow light to pass through it. For instance, a piece of wood is opaque to sunlight, whereas a piece of clear glass is transparent. Even a “translucent” object (like cloudy glass) at least allows light through, which is not the case with an opaque object. At times, the word can be used to describe the reasoning or expressions of someone who is not being very clear in making a point. When someone is less than direct and clear in their speech, they are difficult to understand. This can also happen merely because of the complexity of the subject, which would make it difficult to “see through.”

The adjective "adamant" is etymologically related to the word "diamond"—the hardest stone. Someone who is "adamant" is "hard" in the sense of being relentless, unyielding, or insistent.

The word “dire” is often used in an expression like “dire straits,” meaning “in great distress.” The word generally means “terrible” or “extremely serious.” Among the options given, “dreadful” is the closest to this sense. The word would be used in a sentence like, “The dire situation in American education led the legislature to enact a number of new laws to remedy the terrible performance of the students.”

"Furtive" means secretive or stealthy, typically because of guilt or a belief that discovery would lead to trouble; because of this, the answer choice that is its closest synonym is "clandestine," or kept secret or done secretively, especially because something is illicit.

"Painstaking" is an adjective that means meticulous or extremely careful and particular, so "meticulous" is the correct answer.

“Sacrilegious” means disrespecting religious ideas, objects, or traditions, or in other words, "blasphemous." As for the other answer choices, “serene” means quiet and peaceful; “pious” means deeply religious; “gargantuan” means huge, gigantic, very large, or colossal; and "traditional" means adhering to established conventions without changing them.