Explanation:

- "Harp on" means to talk repeatedly about something, usually in an annoying way.

- Option 1: To comment — Just means saying something; not repetitive.

- Option 2: To criticise — Criticising can be included, but "harp on" is broader and more about repetition.

- Option 3: To keep on talking — "Harp on" means to keep mentioning or talking about the same topic again and again. This is the correct meaning.

- Option 4: To keep on insulting — "Harp on" does not specifically mean to insult, just to mention repeatedly, sometimes negatively but not always.