The correct answer is B: has. This sentence demonstrates a crucial rule of subject-verb agreement involving indefinite pronouns. The word "every" is an indefinite pronoun that always takes a singular verb, even when it appears to refer to multiple individuals. Phrases like "every student," "every person," or "every car" are treated as singular subjects because "every" emphasizes each individual item or person within a group, one at a time, rather than the group as a whole.
Therefore, despite the implication of multiple students, the subject "Every student" functions grammatically as a singular entity. Consequently, it requires a singular verb. "Has" is the singular form of the verb "to have" in the present perfect tense, making it the correct choice to agree with the singular subject "Every student." Options like "have," "had," and "were" are either plural forms or incorrect tenses for this singular subject in the present perfect context. This rule is fundamental for maintaining grammatical correctness in English.