The correct answer is the final option - The adjective phrase incredibly old modifies the word ruins.
A is not correct because very cautiously is an adverb phrase modifying the verb hiked, and not an adjectival phrase. B is also incorrect because the phrase old ruins is not an adjective phrase, but rather a noun phrase. Option C is incorrect because discovered some incredibly old ruins is a verb phrase rather than an adjective phrase.
This leaves us with option D - the phrase incredibly old is indeed an adjective phrase used to modify, or describe the noun ruins. It tells us what the ruins are like.
