Answer should be D, please confirm

The answer should be empirical

Please provide your reasoning. Saying this is correct instead of this does not help the community

The answer is E according to the book. However, for me D could be a good choice.

NOT a good sentence

According the 2nd part of the sentence - despite the story's lack of verifiable sources, hints the 1st part of the sentence aligns with the 2nd part, there is no word hinting towards a contrast,
Aprocryphal - of questionable origin seems most likely
Comprehensible - able to be understood; is not fitting here

Answer consistent to the meaning of the Question must actually be Apocryphal. Comprehensible does not make sense here.

The answer D also makes sense the story is like a myth or a fiction

Explanation

The key clue is the phrase "despite the story's lack of verifiable sources."

If a story has no verifiable sources, it means there is no reliable evidence to confirm that it really happened. The story may be widely told, but its truth is doubtful.

Apocryphal means (of a story or statement) of doubtful authenticity, although widely circulated. This perfectly describes the apple-tree legend-everyone knows it, but historians cannot verify it.

Why the Other Options Are Incorrect

A. humorous - means funny or comical. The lack of verifiable sources has nothing to do with humor.
B. empirical - means based on observation or experiment. This is the opposite of a story lacking evidence.
C. baffling - means confusing or puzzling. The story itself is not described as confusing; the issue is its unproven nature.
E. comprehensible - means understandable or clear. The story is perfectly understandable; that is not the issue.