A registered investment adviser lives in State X. The adviser does business with 1 client in State A and 1 client in State B. The adviser gives seminars about investing to groups of potential customers in State C. The adviser is required to register in:

State X and C

Explanation:

Remember, the clients themselves stay in state A and B, not the investment advisor. According to the requirements of the law, the investment advisor is to register in the state where he gives seminars-State C, and the state in which he resides-X.