The correct option is D but
In the given statement, 'but' is the conjunction, more specifically it's a coordinating conjunction of the adversative/contrasting type. 'But' combines the independent sentences "They had little money" and "they were very content" that are contrast in meaning. "little money" does not imply "very content", as we know most of us aren't content with little money. Hence, option B is correct. 'Very' is an adverb that modifies an adjective, verb or an adverb to emphasize its meaning, here it modifies the adjective 'content'. It does not combine the clauses, hence option A is incorrect, it's not a conjunction. 'They' is a pronoun, a pronoun replaces a noun that's been mentioned before, and 'they' here refers to a group of people. As it does not combine the clauses, it's not a conjunction and option C is incorrect. 'Little' is an adjective that modifies the noun 'money', it's not a conjunction as it doesn't perform the function of combining clauses. Hence, option D is incorrect.