Answer: The purpose of the idiom in the sentence is (D.) to indicate that Tim's success was temporary.
Explanation: The idiom "to be a flash in the pan" is used to make reference to something that happened only once or that did not last much time. In other words, it refers to something or someone that was interesting only for a short period of time. In that way, in the sentence provided, the purpose of the idiom is to indicate that Tim's good performance in the field only lasted for a short period of time and that it is unlikely that he will be successful again in the future.