The books are locked and I can’t ............ them

The phrase “get at” is often used to mean reaching or gaining access to something. Therefore, Option A is the correct answer. 

The other options do not fit in this context.

Option B: “Get to” usually means to arrive at a place or to have an opportunity to do something.

Option C: “Get on” typically means to manage or deal with a situation, especially successfully.

Option D: “Get into” generally means to become involved in a bad situation or to start enjoying something.

Therefore, “get at” is the most appropriate choice here. The sentence would read: “The books are locked, and I can’t get at them.” This implies that the speaker cannot access the books because they are locked away.