Biology question - natural selection?

Some animals, such as mice, are small for security reasons. If confronted by a nasty, mean-minded pussy cat, a mouse can take evasive action by disappearing into a small opening. Being small helps with not being eaten by a largish mammalian predator.

Often, islands don't have any largish mammalian predators, and especially so if the nearest mainland is a boat voyage away. Small fry such as mice, on the other hand, can sometimes accidentally raft in on suitable debris. And should a pair of the complementary sexes arrive on the same natural raft, then they can found a dynasty together; one free from annoyances such as cats.

Given the shortage of predators, disappearing down small openings is no longer a survival advantage. The limit on size is loosened and larger mice can survive to breed as well. That's liable to lead to an average increase in size. And, still being reasonably small, the somewhat restricted availability of food is less of a problem. They don't need that much.

Now consider a population of elephants on Malta. There might not being any there now, but Malta used to have elephants. Their ancestors wandered in when the Mediterranean was on holiday. When the sea returned, the now resident elephants were stranded and food was restricted. That made being large disadvantageous due to food shortage. This was less of a problem for elephants that happened to be smaller.

Malta, and other local islands, ended up with elephants of a large-dog sort of size. The selective pressures of a low food supply meant small is goodiful while elephant big elephants starved. And starving to death plays havoc with your breeding prospects.

Think, for example, that the temperature of the world drops. Those animals who have genetics fit for a warmer climate die off without offspring, while the ones more fit for the cold survive and have offspring. So, the genetic codes that make fur longer, etc., are passed on and become prominent. This is basic natural selection. In a warmer climate situation, the result is the exact opposite. Evolution and natural selection is concerned with the emergence of new species and genetic traits--A only deals with variations in a species. Similarly, D does not have much to do with evolution and the changes in species. B also doesn't seem to imply any genetic change. So, I would choose C because natural selection is responsible for changing the genetics of a population and making it into a different species than it originally was.

Being bigger (on the island) would mean that it is easier to reach high foods and it may be easier to resist harsh wind, but smaller animals on the island would find it easier to find shelter and would not need as much food.