What is the correct adverb to fill the blank: Mr. Transom stared ____________ at the group of students talking in the corner and they shut up immediately.

The correct adverb to fill in the blank is "crossly." Adverbs modify verbs, adjectives, or other adverbs, and in this case, "crossly" describes how Mr. Transom stared. It indicates the manner of his action—showing that he was staring in an annoyed or displeased way. Using this adverb effectively conveys the emotional tone of the situation, which led the students to quiet down immediately due to the intensity of his gaze.

The other options do not fit as adverbs. "Cross" is an adjective that describes a state of being rather than modifying an action. "Crossness" is a noun that represents the quality or state of being cross, which does not apply in this sentence since it requires an action description. "Crossing" is the present participle form of the verb "cross," which also does not function as an adverb in this context. Thus, "crossly" is the only option that appropriately completes the sentence and conveys the intended meaning.