What is the most appropriate ANTONYM of the word ‘fluctuate’?

The correct option is Stabilise.

Step-by-Step Explanation:

1. Understand the meaning of the given word:
The word 'fluctuate' means to rise and fall irregularly in number or amount, or to waver constantly between different states, conditions, or decisions. It implies instability, changeability, and constant movement (e.g., "the stock prices fluctuate daily").

2. Analyze the options:
- Stabilise (British English spelling of "stabilize"): This means to make or become unlikely to change, fail, or decline. It means to make something steady, secure, and constant. Since this is the direct opposite of waving or changing irregularly, it is the antonym of fluctuate.
- Deflect: This means to cause something to change direction; to turn aside from a straight course. It does not mean to stay constant.
- Fickle: This is an adjective meaning changing frequently, especially as regards one's loyalties, interests, or affection. It is a synonym or related concept to fluctuation/instability, not an antonym.
- Vacillate: This means to waver between different opinions or actions; to be indecisive. This is a synonym of fluctuate.

3. Conclusion:
Therefore, the most appropriate antonym (opposite in meaning) of 'fluctuate' is 'Stabilise'.