Answer:
C. Syntax.
Step-by-step explanation:
Syntax can be defined as s set of rules in language. It sets the rules of using words, punctuation, phrases, and clauses that are used to form a grammatically correct sentence.
The sentences stated in the question is an example of the syntax. Both the sentences are grammatically correct according to the rules of syntax. And, if, these sentences were understandable by dolphin then it means that it has the knowledge of syntax.
So, the correct answer is option C.