Great question. I think your initial interpretation is correct, minority is less than half, thus it is less than most and is best expressed as some. We can diagram the first sentence as:
PA some-> SJ
but notice that the natural language of this sentence suggests that if only a minority engage out of a sense of social justice, the majority must engage for other reasons, thus we can also diagram the first sentence as:
PA most -> ~ SJ
The invalid conclusion that follows can be diagrammed as:
SJ some -> ~PA
The argument in the answer choice (A) follows the exact same structure:
S most -> ~PP PP some -> ~S
Does this help? Let me know if have any further questions.
