Answer:
C. Participants in the seminar will hear lectures on a variety of topics: building professional networks, managing retirement portfolios, and maximizing career opportunities.
Explanation:
In English language, colons introduce clauses/sentences which illustrates, explains, expands, or throws more light on the information/sentence that preceded the colon.
They may be used between independent clauses when the second sentence explains the first sentence.
The answer above informed us what participants will hear at the seminar with the help of a colon.