In Risk Management, how is the term "likelihood" related to the concept of "threat?"

Likelihood is the likely source of a threat that could exploit a vulnerability.

Likelihood is the probability that a vulnerability is a threat source.

Likelihood is the probability that the threat-source will exploit a vulnerability.

Likelihood is a possible threat-source that may exploit a vulnerability.

Explanation

Likelihood is a per threat measurement and it's based on how likely the threat is to exploit a known vulnerability. This question has two possible answers-- What is the difference between source of a threat and threat-source? They are the same thing.
I don't agree with the answer. Since Threat is Possibility that a vulnerability can be exploited. Risk is a probability to exploit vulnerability.

I think it should be answer A - Probability of something happening, e.g. "threat-source will exploit a vulnerability". Answer B says "a possible threat-source that may exploit" describes an object, not a probability. From ISO 31000 2018 (https://www.iso.org/obp/ui#iso:std:iso:31000:ed-2:v1:en) - section 3.7 "likelihood - chance of something happening Note 1 to entry: In risk management (3.2) terminology, the word “likelihood” is used to refer to the chance of something happening, whether defined, measured or determined objectively or subjectively, qualitatively or quantitatively, and described using general terms or mathematically (such as a probability or a frequency over a given time period)."