Regarding
Assertion[A]: A peacock has a more vibrant and beautiful plumage when compared to a peahen.
Reason[R]: Sexual selection leads to the exaggerated dimorphic traits that are used in competition over mates.
Both [A] and [R] are true and [R] is a correct explanation to [A]
Sexual dimorphism, the differences in appearance between males and females of the same species, such as in colour, shape, size, and structure is quite common among organisms that reproduce sexually.
The colorful feathers or plumage of a male peacock are used a as part of a courtship ritual in the hope of attracting the female's or peahen’s attention. The peacock with the best plumage often succeeds in being chosen as the mate. This is a clear example of a dimorphic trait being used to gain an advantage for sexual reproduction.