Answer:
The pod of whales surrounded their prey before attacking.
Step-by-step explanation:
A sentence is said to be grammatically correct when it has subject-verb agreement, makes correct use of punctuation and is unambiguous.
With this in mind, the sentence from the given options that is grammatically correct is option C.
This is because it has a clear meaning and obeys all the other rules of grammar while the other options have problems with ambiguity and subject-verb agreement.