Final answer:
The addition rule(option a) of probability is used for analyzing genetic crosses with unordered combinations of outcomes, by adding the individual probabilities of each event.
Step-by-step explanation:
To analyze genetic crosses in which the problem has an unordered combination of outcomes, the appropriate operation of probability to use is the addition rule. This rule is utilized when dealing with two or more events that can occur in combination, and we are interested in the probability of either event happening. The addition rule states that the probability of occurrence of any one of several mutually exclusive events is found by adding their individual probabilities.
For example, in a genetic cross where we want to know the probability of an offspring inheriting at least one dominant trait from a dihybrid cross, we use the addition rule to sum the probabilities of each event where a dominant trait appears.