The correct option is
A In
The given sentence should be filled with a preposition of time since it speaks about summer. Thus option D is incorrect, since ‘of’ is not a preposition of time. The preposition ‘on’ is used to indicate a day or date, while ‘at’ is generally used while speaking about a specific time. ‘Summer’ doesn’t come under both of these categories, it neither specifies a day/date nor gives an exact time at which the action takes place. Hence options B and C are incorrect. Option A is the correct answer.