The correct answer is A. Confused
Step-by-step explanation:
Participles are types of words that derive from verbs and often end in "-ing", "-d" or "-ed", although this varies in irregular verbs. Besides this, in grammar participles act either as part of compound verbs or as adjectives by defining nouns. In the case of the sentence "The confused student shook his head to clear it" the word that is a participle is "confused" because this word derives from the verb "confuse", end in "-ed" as most participles and also acts as an adjective by describing the noun "student".