When Callum talks to anyone, whether it is at his company functions, at his sister's wedding, at his college reunions, or on social sites, he is ________.

A) using persuasive communication
B) expressing himself
C) building speaking skills
D) networking

The given scenario describes Callum engaging with people in different settings and contexts, suggesting that he is actively building and expanding his network. The other options of persuasive communication, self-expression, and speaking skills may also be relevant to Callum's interactions, but networking appears to be the most appropriate answer based on the given information.