The things we do are never completely our own; the brain itself is a/an culturally organ, shaped by social interact.       

The first emboldened word must be an adverb, which qualifies the adverb 'never'. According to this, options a) and c) are negated because these are adjectives. The second emboldened word must be an adjective which qualifies the noun 'organ'. According to this, the emboldened word is negated because it is an adverb. The third emboldened word must be a noun which can be qualified by the adjective 'social'. According to this, the emboldened word and option d) are negated because these are verbs. Option b) satisfies all the grammatical conditions mentioned above and brings out a meaningfully coherent sentence. Hence, b) is the correct answer.