The most appropriate "one word" for a professional rider in a horse race is "Jockey."

Explanation of the options:

☀ Horse Pilot: This term does not accurately describe a professional rider in a horse race. A horse pilot is not a commonly used term in this context.

☀ Cellar: This term refers to an underground storage area, typically for wine or other goods. It does not relate to a professional rider in a horse race.

☀ Jockey: A jockey is a professional rider who competes in horse races. This is the correct and most suitable term for the given expression.

☀ Autocrat: An autocrat refers to a ruler with absolute power and authority. It does not relate to a professional rider in a horse race.