Option A: 'Beneficial' is the adjective form of the verb 'benefit'. For example, 'The beneficial effects of coconut oil.', where 'beneficial' modifies/describes the noun 'effects'. Hence option A is correct.
Option B is incorrect because it's the adverb form of the verb. It describes the manner in which something is done.
Option C is incorrect because the word 'beneficialism' does not exist in standard English.
Option D is a noun, hence it's not the adjective form; it's incorrect.