The correct option is A If Margaret hadn't worn a seat belt, she would have been injured.
The given situation is real-it states something that happened.
The if-sentence states a hypothetical situation, something that could have happened in place of what actually happened.
Option A: This sentence is a type 3 conditional sentence. It states a hypothetical past event, with its probable result in the past.
The sequence of tense is: if-clause (past perfect tense) + main clause (perfect conditional)
It is relevant to the given sentence in the question.
Also, the given sentence is in past perfect tense 'had worn', hence the conditional verb would also be in past perfect tense 'hadn't worn'.
So option A is correct.
Option B: 'Had been wearing' is in the past perfect continuous tense. An if-clause is generally not in this tense form.
Option C: 'Was wearing' is in past continuous tense.
Option D: Similarly, 'was not wearing' also states a continuous tense.
If-clauses do not state the continuous form.
Hence options B, C and D are incorrect.