The claim about diet affecting heart attacks is not supported by anything else in the argument, so cannot be a subsidiary conclusion.

As with (C), the author never makes a claim about when we should or shouldn’t hold someone responsible. The argument is just trying to show that we assess responsibility differently in different situations.

As with (B), the author makes no statement about how we should assess responsibility. The point of the argument is just to demonstrate that our assessment can differ based on factors other than free will.

The author isn’t trying to disprove anything, and the argument never references a claim that we shouldn’t hold criminals morally responsible.

This accurately describes what the claim about diet and heart attacks does in the argument: it’s a premise. It supports the conclusion as part of one of two conflicting examples.