Crows appear to behave ………………, actively helping one another to find food.
The phrase or the clause found in the sentence parenthetically next to the missing word will elaborate the meaning of the missing word.
You have to identify the single word which means the same as the part of the sentence which elaborates its meaning.
What is the single word that stands for the quality of actively helping one another top find food?
The obvious word is cooperation. Since, cooperatively is one among the given choice, you can choose that as the answer.
So, the answer is (D).
GRE : Model Questions Index