The correct option is
A He was the only person who wasn't killed in the crash.
The given sentence means that the subject was the only person that survived the crash. In the answer, the subject(Peter brown) and the object(crash) remains the same. Thus, the rest of the phrase must mean 'only person that survived'.
Option A is correct. Here' the only person who wasn't killed' means the same as 'the only person who survived. Thus, it has the same meaning as the required one and is the correct answer.
Option B is incorrect. It means that the subject was the only person who did not survive the crash. Thus, it means the opposite of what is required and so, it cannot be the answer.
Option C is incorrect. It means that the subject was not in the crash at all and that he was the first person to reach the scene of the crash. Thus, it does not have the required meaning and is incorrect.
Option D is incorrect. It means that the crash happened only because of the subject. Thus, it does not have the correct meaning and is incorrect.