The correct answer is C: is. This question focuses on subject-verb agreement with collective nouns. A collective noun, such as "flock," "team," "committee," "family," or "audience," refers to a group of individuals. When the group acts as a single, unified entity, the collective noun takes a singular verb. In this sentence, "the flock of birds" is acting together as one unit, flying in a single direction.
Consider these points:
- Unified Action: The birds are flying together as one cohesive group, hence the singular verb "is."
- Collective Noun Rule: When a collective noun emphasizes the group as a whole, it requires a singular verb.
If the sentence implied that the individual members of the flock were acting separately or disagreeing, a plural verb might be used (e.g., "The flock are scattering in different directions"). However, "flying south" indicates a unified action. Options A ("are"), B ("were"), and D ("have") are all plural verb forms and are incorrect because "flock" is treated as a singular unit in this context.