Explanation:

- To sit on the fence

- This idiom means to avoid making a decision or choice. It is often used when someone is neutral or undecided in a situation.

- Option:1, to act promptly

- This means to act quickly and without delay.

- It does not relate to indecision or neutrality.

- Option:2, to be lazy

- This refers to being idle or unwilling to work.

- It doesn’t indicate indecision or neutrality.

- Option:3, to be undecided

- This matches the idiom’s meaning of avoiding a firm decision or stance.

- It conveys neutrality or hesitance in making a choice.

- Option:4, to sit without doing anything

- It implies inactivity, but not necessarily due to indecision.

- More about passivity than avoiding a decision.