He had HepB at 1 day of age at 8 weeks he had HepB, DTap, Hib, IPV, and PCV.

1. This is a fragment that requires a verb.
2. This is a comma splice that can be fixed by changing the comma after HepB to a semicolon.
3. This is a run-on that can be fixed by adding a semicolon after age.
4. This is a run-on that can be fixed by adding a period after the first mention of HepB.
5. The sentence is fine as it is.
Answer:
Definately not #5. #4.
My answer is # 3 - adding a semicolon after age.

Right again.

Great job! You correctly identified that the sentence is a run-on and needs to be fixed. Adding a semicolon after "age" is the correct way to fix this run-on sentence. By doing so, you are separating the two independent clauses within the sentence and making it grammatically correct. Well done!