If the current gain is 200 and the collector current is 100 mA, the base current is A. 2 mA B. 0.5 mA C. 2:00 AM D. 200 A E. None of the above

The correct answer is $\boxed{\text{B. 0.5 mA}}$.

The current gain of a transistor is the ratio of the collector current to the base current. In this case, the current gain is 200, which means that for every 1 mA of base current, there will be 200 mA of collector current. So, if the collector current is 100 mA, then the base current must be 0.5 mA.

Option A is incorrect because 2 mA is not a multiple of 200. Option C is incorrect