The correct answer is C. An employee of the bank stole the money.
The other options are incorrect because they do not use the past tense correctly. Option A, An employee of the bank had stolen the money, is in the past perfect tense, which is used to talk about something that happened before another event in the past. In this case, the event that happened before the money was stolen is not mentioned, so the past perfect tense is not appropriate. Option B, An employee of the bank steal the money, is in the simple present tense, which is used to talk about things that happen regularly or repeatedly. In this case, the money was stolen only once, so the simple present tense is not appropriate. Option D, The money can be stolen by an employee of the bank, is in the passive voice, which is used to focus on the action rather than the person who is doing the action. In this case, the question is about who stole the money, so the passive voice is not appropriate.
The sentence “The money was stolen by an employee of the bank” is in the simple past tense, which is used to talk about things that happened in the past. In this case, the money was stolen at a specific time in the past, so the simple past tense is the most appropriate tense to use.