Fill in the blank with the most appropriate word.
He stood looking _____ the picture for a long time.
A preposition is a word which comes before a noun or a pronoun and establishes a relationship between the elements of a clause or words.
The correct answer is option D) at as it provides an answer to what was he was looking at ( the picture).
'Through', 'for' and 'after' cannot be used here as they indicate of an activity from the beginning to the end, purpose, and something next in place respectively.
He stood looking at the picture for a long time.