Determine all homomorphic images of $D_4$ up to isomorphism.

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Determine all homomorphic images of $D_4$ up to isomorphism.

What exactly is a homomorphic image without mentioning a second group? Isn't a homomorphism a map between two groups?

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Yes, you need a second group $G$ to have a homomorphism $\phi\colon D_4\to G$. The question asks only for the image of such a homomorphism, hence we may as well assume that $\phi$ is onto. So to rephrase the question:

Find all groups $G$ (up to isomorphism) such that there exists an onto homomorphism $D_4\to G$.