The correct answer is D. risk.
A risk is the chance of something happening that could have a negative effect. It is often measured as a probability, or the likelihood of something happening. For example, the risk of getting into a car accident is the probability of being in a car accident.
A chance is the possibility of something happening. It is often used to describe something that is unlikely to happen, but could still happen. For example, you might say that there is a chance of rain today.
An event happening is simply an event that has occurred. It is not necessarily a negative event. For example, you might say that the event of your birthday happening is a happy event.
Probability is the likelihood of something happening. It is often measured as a number between 0 and 1, where 0 represents no chance of something happening and 1 represents a certain chance of something happening. For example, the probability of getting heads when flipping a coin is 0.5.