= 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

A cost-benefit analysis in my opinion is done in order to take a decission whether implementing a mitigation control would be profitable to reduce the rist to an acceptable level. In this case the decision has alread been taken and controls have been implemented, so to actually evaluate whether the implemented controls were indeed effective to reduce the rist to an acceptable level is to do a risk assessment to evaluate the current risk.

A successful risk management practice minimizes the residual risk to the organization. Choice A is incorrect because the fact that overall risk has been quantified does not necessarily indicate the existence of a successful risk management practice. Choice B is incorrect since it is virtually impossible to eliminate inherent risk. Choice D is incorrect because, although the tying of control risks to business may improve accountability, this is not as desirable as minimizing residual risk.