Can you fill in the blank?
Because bone loss occurs earlier in women than ________, the effects of osteoporosis are more apparent in women.

Because bone loss occurs earlier in women in men, the effect of osteoporosis are more apparent in women.

In men

From the start you should notice this sentence is comparing. The blank is after than so you need the second comparison element. Option A is eliminated because is has the auxiliary; which is unnecessary. Option C and D are eliminated because they complete another type of comparison. Leaving option B as the correct option [preposition + noun]