The correct answer is: A. in
The preposition “in” is used to indicate a position within a larger space. In this case, the blonde girl is standing in the first row.
The preposition “on” is used to indicate a position on top of something. In this case, the blonde girl is not standing on top of the first row.
The preposition “at” is used to indicate a position near something. In this case, the blonde girl is not standing near the first row.
The preposition “over” is used to indicate a position above something. In this case, the blonde girl is not standing above the first row.