Which is a fused sentence?

A. Juanito likes to play drums that he makes out of old cans.

B. I enjoy playing video games because they are so exciting.

C. My dad doesn't have a hobby he is too busy working.

D. Felicia likes to watch old movies and listen to old rock music.

The option C is a fused sentence, because its connecting two "parts", "clauses" or "phrases" without being separated from punctuation or other connectives, that should make it a proper sentence. Therefore, we could make various types of changes to turn this fused sentence into a correct sentence:

Addition of a semi-colon (; )

My dad doesn't have a hobby; He is too busy working.

Using a subordinate conjunction

My dad doesn't have a hobby because he is too busy working.

Separating definitely the clauses:

My dad doesn't have a hobby because he is too busy working.