The correct option is C Came by
Phrasal verbs are phrases, formed with a verb and a preposition or an
adverb. The meaning thus formed is different from its literal meaning.
Option D: 'Came by' is a phrasal verb. It means 'to gain possession of'.
It fits the sentence in meaning, that 'he gained possession of a huge treasure during his travels'.
Hence option D is correct.
Option A: 'Came into'. If you 'came into something', it means that something became yours after its previous owner died.
The meaning isn't relevant to the sentence, as it does not mention someone's death.
Option B: 'Came to'. If something came to you, it means that you thought of it or remembered it. For example, 'the idea came to me while I was away on vacation'.
This phrase, too, does not
complete the meaning of the sentence.
Option C: 'Came for' is not a phrasal verb and does not complete the sentence in meaning.
Hence options A, B and C are
incorrect.