Explanation:

The correct option to fill the blank is while.

Meaning:
The phrase "make hay while the sun shines" is an idiomatic expression that means to take advantage of an opportunity while it is available.

Explanation:
- "Make hay" is a metaphorical phrase that means to take advantage of a situation or make use of an opportunity.
- "While" is used to indicate a period of time during which something happens.
- "The sun shines" refers to a favorable or advantageous condition or situation.

Example:
- This phrase is often used in the context of taking action or making progress when conditions are favorable.
- For example, if there is a limited time to complete a project or task, it is important to work diligently and efficiently while the opportunity is available.

Usage:
- The phrase "make hay while the sun shines" is often used to encourage people to make the most of their opportunities and not to procrastinate or delay action.
- It emphasizes the importance of taking advantage of favorable conditions and not waiting for a more convenient or comfortable time.

Synonyms:
- Seize the day
- Strike while the iron is hot
- Carpe diem (Latin for "seize the day")

Conclusion:
The correct option to fill the blank is "while," as it accurately completes the idiom "make hay while the sun shines" and conveys the meaning of taking advantage of an opportunity.