'Idiomatic expressions are a
type of informal English that have a meaning different from the meaning
of the words in the expression.'
"Flash in the pan" is an idiomatic expression that means: 'someone whose success and popularity is short-lived'.
Option D gives a similar explanation for the idiom. Also, it
relates to the
given sentence: that his winning the talent show was not short-lived.
Hence D is the correct option.
Option A also gives an explanation, but it is incomplete. It does refer to a noticeable achievement, but it doesn't refer to it as short-lived. This meaning 'achievement that does not last long' is essential to the idiom.
Option A does not express the complete meaning.
B and C: These options each do not give the correct explanation for the idiomatic
expression. They give meanings that are derived from this expression.
Hence options A, B and C are incorrect.