Which of the following variations could be subject to natural selection?

Which of the following variations could be subject to natural selection?
a) A dog with short legs due to malnutrition is able to crawl into holes better than his litter matesb) A tree is not infested by a ground-dwelling beetle when the homeowner cuts the lower branchesc) A hyena is born with a spotted fur pattern that allows it to hide in the grass better than his litter matesd) A pigeon learns than when its keeper comes near it will be fede) All of these variations may be acted on by natural selection