Here are the articles filled in for each sentence:

1. Can I have **an** apple?
2. Have you ever seen **the** Acropolis in **Athens**? (no article needed for "Athens")
3. She arrives at **the** college by **a** taxi in **the** morning on **the** stroke of nine.
4. **Influenza** is **an** acute disorder caused by **a** virus, often giving rise to **widespread** epidemics. (no article needed for "influenza" and "widespread")
5. **The** Monday after should be **a** suitable day for **the** launching **of the** boat.

Let me know if you need further assistance!