What can lead to reproductive isolation after just one generation? a. polyploidy c. changed allele frequencies b. increased mutation rate d. independent assortment of chromosomes?

The answer is increased mutation rate.
Reproductive isolation is the existence of biological factors that impede members of two species from interbreeding and producing viable fertile offspring. These biological barriers may be pre-zygotic or post zygotic; post zygotic barriers include developmental errors of embryo, become infertile, or not living long enough to reproduce; while the prezygotic barriers include; habitat isolation, temporal isolation, genetic barriers among others. 

It should be cryosphere and hydrosphere