Which word is the subject of the sentence? In the elm tree cawed one large black crow. A. tree B. an understood you C. crow D. black

Actually, the crow is the subject!

the structure of the sentence is like this:

In the elm tree (where? we can substitute this phrase with "there")

cawed - this is the verb of the sentence

one large black crow - this is the full subject of the sentence, let's substitute this sentence with "it"

then we have a sentence "there cawed it" - now it's easier to see that the "it" must be a subject.

then in the phrase

one large black crow

the head is Crow - that's the correct answer - and the rest of the words are adjectives to give more information about this Crow.