If a high school offers a myriad of classes, it means that there is a vast amount or a seemingly innumerable number of classes.

The word “myriad” is derived from the Greek for "10,000." In general, it is used to describe a countless multitude of things. For that reason, the best option among the provided answers is “innumerable.” An example usage would be, “Roger wished to do something so foolish that his friends were able to provide him with myriad reasons, overwhelming him with reasons for making such a stupid choice.”

You might be more familiar with the word “platitude” than with “platitudinous.” The former is the related noun form of the same base. A “platitude” is an expression, often moral in nature, that has been so overused that it has lost its meaning. You might know someone who recites such expressions as, “Everything happens for a reason.” Such remarks either tell you nothing or often just frustrate you when horrible things happen. They provide little consolation or direction. Although a “platitudinous” expression could be called “annoying,” it is most properly said to be something “overused.” The annoyance follows on the fact that overuse has totally shorn it of its meaning and significance.

"Replete" and "abounding" both mean full or well-stocked. "Scant" means meager or paltry. "Disingenuous" means insincere or deceitful. "Evanescent" means transient or brief. "Pugnacious" means belligerent or aggressive.

"Aesthetic" describes something having to do with beauty. Be careful not to confuse it with similar sounding words like "ascetic" (harshly disciplined) or "aseptic" (sanitary).

"Tawdry" and "tacky" both mean cheap or tasteless. "Sophisticated" means refined or elegant. "Nebulous" means confused or obscure. "Taciturn" means uncommnicative. "Urbane" means cosmopolitan or sophisticated. 

"Tepid" takes on one of its two meanings in this question. "Tepid" can mean lukewarm, but it can also mean halfhearted or indifferent. This question uses the secondary meaning of tepid and the best answer choice is halfhearted.

The word “edifying” is related to the English “edifice,” meaning building. Most strictly, “to edify” means to improve someone intellectually or morally. It is often used in the sense of “improvement” in general, though this is a secondary usage. Bearing this qualification in mind, “improving” is still the best option among those given. One can speak of “edification” when describing the instruction itself, as in, “He provided edification to the young man in order to improve his moral behavior.”

Something that is "garish" is excessively and crudely showy. "Gaudy" carries the same sense of being tastelessly flashy. "Modest" is an antonym.

If something is benign such as the weather, it indicates that it is mild.  In the case of weather, benign conditions are the opposite of inclement weather.