The most obvious difference among the answer choices is "with" and "of." Using "of" after "an enormous scale" makes it sound like the 24-country board is what makes the scale enormous. In fact, the 24-country board is how the IMF manages economic policy; the board does so "with" the IMF. That leaves us with (A) and (B).
The difference between those two choices is at the end: "and it includes" or "including." "and it includes" makes "includes" parallel to "attempts," implying that the IMF includes Montenegro. That is vague; better to use the construction in (B), which implies that Montenegro is one of the 185 member states. Choice (B) is correct.