Select the most appropriate meaning of the underlined phrase.
Tanim’s parents do not like Mohon, but he keeps meeting with him on and off.

On and off (Phrase)
English Meaning: Intermittently
Synonym: periodically, at intervals, off and on, (every) once in a while, every so often
Example Sentence: It rained on and off most of the afternoon
Hence, option 3) conveys the correct meaning of the underlined phrase.

Source: Oxford Dictionary