I would say that all answers apply.
A) If the phenotype is genetically determined, the frequency of the alleles influencing that phenotype should impact the prevalence of that phenotype.
B) Selection can either negatively or positively affect the frequency of a phenotype by decreasing or increasing the survival rate and lifetime reproductive fitness of an individual with said phenotype.
C) If a migration into or out of the population contains enough individuals with a specific phenotype relative to the remaining population, that can also change the frequency of the phenotype.
D) If a phenotype is involved in sexual selection, the randomness of mate choice can come into play. If mates are chosen randomly, then the phenotype should not be affected by mate choice. However, if the mating is not random and that phenotype has an impact on mate choice, its frequency can increase or decrease depending on how that phenotype affects mate choice.
