The adjective “vitriolic” describes a kind of speech that is critical not in there merely “evaluative” sense but instead in a nasty and spiteful manner. An example use of the word would be, “She did not merely speak mild words to her adversary but instead, in great anger, shouted vitriolic condemnations against him.”

A thrifty person is very careful not to waste resources. While it might be tempting to assume that such a person is greedy or avaricious, the word only expresses the fact that he or she “pinches pennies,” that is, is very careful with any outlays. Note that someone can be “thrifty” with any resources, though the term is most often used to describe care with regard to money. The word “frugal” is the only option that expresses this notion of thrift.

The word “frugal” is related to base words in Latin for “fruit.” It means “being economical” or careful with resources. When someone is “thrifty,” he or she is careful in spending money. A “thrift store” is generally a second-hand store meant to allow people to save money on products that can be purchased at a lower-than-normal price because they are second hand.

The word “staunch” has two meanings. One of them is “faithful” or “devoted.” This is the sense that is found here among the options provided. You might often hear the expression “staunch supporter.” Such a person is not an occasional supporter but instead is one that stands with whatever the cause / person is “come thick or thin.” The word can also mean “to stop the blood coming from a wound.”

Typically, someone who is hostile and aggressive is characterized as belligerent.

"Irascible" means cantankerous or crabby. "Tenacious" means strong-willed or determined. "Wary" means careful or cautious. "Opaque" means clouded, muddy, or hard to understand. "Mirthful" means merry or playful. 

"Saccharine" means something cloying or sickeningly sweet. "Munificent" means generous or giving. "Abashed" means confused or embarassed. "Mundane" means ordinary or humdrum. "Frigid" means frozen or exceptionally cold.

You might confuse “prodigal” with “prodigy” or “prodigious.” The former means “being very talented,” especially when that person is very young. For instance, one can speak of Mozart as being a “child prodigy,” having shown great musical talent from his youth. Something “prodigious” is impressive and of great magnitude. When something is “prodigal,” it is “great” in the sense of being wasteful. The Biblical story about the “Prodigal Son” is about a son who spends all of his money wastefully before coming home to seek forgiveness. He is called “prodigal” because of this wastefulness.

"Sagacious" and "astute" both mean smart or judicious. "Arrogant" means haughty or overly proud. "Barefaced" means shameless or open. "Complaisant" means agreeable or easy-going. "Effluent" means drainage or outflow. 

“Imperious” means overbearing, domineering, and assertive, so "overbearing" is the correct answer.  “Underneath” means under or beneath; “detached” means not connected to when describing physical objects and dispassionate when describing people; “abashed” means embarrassed; and "military" means having to do with a country's official armed forces.