Question

Which of the reproductive isolating mechanisms given below is postzygotic?

Answer 1

Postzygotic means after the zygote, or fertilized egg, has formed. Therefore, the mating season is not a factor because mating has already happened, and the same is true of pollination, which is mating in higher plants. Wrong pollen will not fertilize the wrong flowers because the DNA will not match.
Therefore, the answer is Hybrids form but are sterile.
Hybrids can be formed by breeding two closely related species, but most of them are sterile. They will grow and thrive from their zygote state, but they cannot reproduce themselves.
With animals, a common example is mules. A donkey and a horse are mated, and a mule results. It is larger and stronger than a donkey but more sure-footed and with stronger legs than a horse. However, mules are female and sterile, so to get more mules you must breed more horses with more donkeys.
With plants, farmers do this all the time. Two different species of corn, for example, can be crossbred to produce sweeter, more tender corn kernels, but the corn plants will not produce seeds because the hybrid corn is sterile, so farmers must purchase new hybrid seeds each season from companies which specialize in crossbreeding different species of plants, either for food or for flowers that are more appealing to the customers than either of the individual species from which the hybrid is bred.