Answer:
C. Phatic
Step-by-step explanation:
The phatic function of a language means to initiate a conversation with a person or a group of people. Only the people that have been given the word are acknowledged to be a part of the conversation. The words used may not have content itself but is used for the reason mentioned above.
The words "What's up" does not have meaning of its own but is used to know who is part of the conversation.
Hence, "What's up" represents a language's phatic function.