Preposition 13

 He isn't good _____ French.

When a word is placed in front of a noun or pronoun to show in what relation the person or thing denoted by it stands in regards to something else is a preposition. In this case, the blank is in front of a pronoun, so we would be identifying a preposition. When the preposition is placed before an object it is usually a preposition of place or a position of someone. 

We use "of" when we want to show that people or things relate to other thing. It is the wrong option 

We use "at" to specify a particular place or position of someone. It is the correct option.

We use "in" when we mention about time. It is the wrong answer

We use "to" to simply “point at” or “activate” something. It is the wrong option

Option A is correct.