= According to the CISM Manual1, risk is defined as the combination of the probability of an event and its consequence. Therefore, determining the risk for a particular threat/vulnerability pair before controls are applied can be expressed as a function of the likelihood and impact, should a threat exploit a vulnerability.
Likelihood is the probability or frequency of a threat occurring, while impact is the magnitude or severity of the harm or loss that would result from a threat exploiting a vulnerability. The higher the likelihood and impact, the higher the risk. The lower the likelihood and impact, the lower the risk.
The other options are not correct because they do not capture the full expression of risk. Option B only considers the impact, but not the likelihood, of a threat exploiting a vulnerability. Option C confuses the risk with the risk response, which is the action taken to reduce or mitigate the risk. Option D only considers the likelihood, but not the impact, of a threat attempting to exploit a vulnerability.
References = CISM Manual1, Chapter 2: Information Risk Management (IRM), Section 2.1: Risk Concepts2
1: https://store.isaca.org/s/store#/store/browse/cat/a2D4w00000Ac6NNEAZ/tiles 2: 2