To understand what determines the level of risk, the user needs to know what risk is and how it is measured. In general, risk is the likelihood that an event will occur and its potential impact.

Now, let's go through each option and explain why it is right or wrong:

A. The cost of dealing with an adverse event if it occurs: This option is partially correct but incomplete. The cost of dealing with an adverse event is a factor in determining the level of risk, as it is related to the impact of the event. However, it does not take into account the likelihood of the event occurring.

B. The probability that an adverse event will occur: This option is partially correct but incomplete. The probability of an adverse event occurring is a factor in determining the level of risk, as it is related to the likelihood of the event. However, it does not take into account the potential impact of the event.

C. The amount of testing planned before release of a system: This option is incorrect. While testing is an important aspect of risk management, the amount of testing planned does not determine the level of risk. Testing helps to identify and mitigate potential risks, but it does not impact the inherent risk level.

D. The likelihood of an adverse event and the impact of the event: This option is correct. The level of risk is determined by both the likelihood of an adverse event and the potential impact of the event. The likelihood and impact are often combined to create a risk rating or score, which is used to prioritize risk mitigation efforts.

Therefore, the correct answer is: D. The likelihood of an adverse event and the impact of the event.