Jevais à Lagos _____ les vacances.

Explanation

The correct answer is pendant
The correct sentence would be: "Je vais à Lagos pendant les vacances." Which translates to "I am going to Lagos during the holidays." "Pendant" is a preposition that indicates an action that occurs during a specific period of time, in this case, the holidays.
Note: The other options don't fit the context:
"Contre" would indicate opposition or against "Depuis" would indicate a starting point or since "Par" would indicate a means or agent
"Sous" would indicate a location under something.