Special mating plumage produced by birds during a time of year is an example of:

a reproductive isolating mechanism

winter coloration to blend into the background

as aspect of aging of the birds as they become old and senile

none of these

None of these is true.

Notice how it says "special mating plumage" key word "mating". Birds utilize a certain plumage in order to win mates, for example birds of paradise (aside from the interesting mating behavior).

Reproductive isolation might sound correct but it is not, what it means is the species can not reproduce with other species of animals.