"Aversion to risk," "aversion of risk," and "risk averse"

By far the most idiomatic way to express this idea is risk averse.

Elmira asks a good question about which idiom to use.

Aversion to risk is idiomatic, but rare:

If you are an investor with an aversion to risk, you should choose index funds.

It is possible to use aversion of risk, but it is not very idiomatic.

The most common way to express this idea is with the phrase risk averse:

Most investors over the age of 60 are risk averse. [=afraid of risk]

My friend goes on adventurous vacations, but I'm too risk-averse to go with him.