“Efficacious” means successful in producing a desired or intended result; effective, hence Option C is the correct answer because “effective” means successful in producing a desired result, hence it is the correct answer.
Option A is incorrect because “suitable “ means right or appropriate for a particular person, purpose, or situation, which is not the correct meaning of efficacious hence incorrect.
Option B is incorrect because “possible” means something that has a probability of being done, which is not related to efficacy hence incorrect.
Option D is incorrect because “proper” means something that Is genuine hence incorrect.