The idiom "off and on" (or "on and off") means doing something intermittently or irregularly, not continuously. The best synonym among the options is (C) Occasionally, as it conveys an action that happens from time to time rather than constantly.
Meaning: intermittently, from time to time, sometimes, irregularly.
Example: "She works as a tutor off and on" means she works occasionally, not on a regular schedule.