The correct answer is C. My dad doesn’t have a hobby he is too busy working.
Step-by-step explanation:
A sentence is considered as fused if the sentence contains two independent clauses together without proper punctuation such as a semicolon or a transition. This type of grammar error can be seen in "My dad doesn’t have a hobby he is too busy working" because in this sentence there are two independent sentences each with a subject, verb and even object/complement "My dad doesn’t have a hobby", and " he is too busy working" but there is no punctuation or transition between them.