Final answer:
One major disadvantage of joining an intergovernmental organization (IGO) for member countries is the difficulty in convincing member countries to cooperate with the IGO's decisions. So, the correct answer is option d.
Step-by-step explanation:
One major disadvantage of joining an intergovernmental organization (IGO) for member countries is that it can be difficult to convince member countries to cooperate with an IGO's decisions.
Member countries may have their own desires and interests that they prioritize over the goals of the broader international community.
For example, powerful states within the IGO may have disproportionate influence over its actions, which can lead to decisions that favor their own preferences and disadvantage other states.
So, the correct answer is option d.