Explanation:

- The phrase "keep on" typically means to continue doing something.

- Option 1: Continue

- This is a direct synonym for "keep on," meaning to persist in an action or process.

- Correct Answer

- Option 2: Place on

- This refers to the physical act of putting something on a surface.

- Not relevant to the meaning of "keep on."

- Option 3: Put on

- This often means to dress oneself with clothing or apply something.

- Not related to continuing an action.

- Option 4: Start

- This indicates the beginning of an action or process.

- It's the opposite of continuing an action.