The correct answer is: B. y is true when x is known to be true.
Rule confidence is a measure of how likely it is that a rule is true. It is calculated by taking the probability of the consequent (Y) given the antecedent (X). In other words, it is the probability that Y will occur if X is known to be true.
For example, consider the rule “If it is raining, then the ground is wet.” The rule confidence for this rule would be the probability that the ground is wet given that it is raining. This probability would be high, since it is very likely that the ground will be wet if it is raining.
The other options are incorrect because they do not measure the likelihood that a rule is true. Option A, y is false when x is known to be false, measures the likelihood that Y will not occur if X is known to be false. Option C, x is true when y is known to be true, measures the likelihood that X will occur if Y is known to be true. Option D, x is false when y is known to be false, measures the likelihood that X will not occur if Y is known to be true.