Answer:
C. It is missing a subject noun or pronoun
Step-by-step explanation:
This sentence is not complete because it does not include:
-A pronoun (he, she, it, they, I, we, no one, etc)
OR
-A subject noun (Jack, The cat, Tom, etc)
Therefore the sentence must include one in the blank to be a complete sentence
_____ yelled "No!" at the dog before it could attack.
The action is yelling.
Without filling in the blank, no one knows who is completing the action.
The dog is receiving the action (yelling of "no")
This all prevents the attack.
The answer is C.
hope this helps :)