The correct answer is C.
To go off on a tangent is to start talking about something that is not directly related to the topic at hand. It can be done accidentally or intentionally, and it can be frustrating for the listener if it happens too often.
Option A is incorrect because changing the subject immediately is not the same as going off on a tangent. Changing the subject is a conscious decision to talk about something else, while going off on a tangent is more likely to be an unintentional lapse in focus.
Option B is incorrect because forgetting things in between is not the same as going off on a tangent. Forgetting things can happen to anyone, and it does not necessarily mean that the speaker is not interested in the topic.
Option D is incorrect because becoming boisterous is not the same as going off on a tangent. Becoming boisterous is a change in behavior, while going off on a tangent is a change in topic.
In conclusion, the correct answer is C. He is an interesting speaker but tends to go off at a tangent, meaning that he starts talking about something that is not directly related to the topic at hand.