In a particular population of mice, certain individuals display a phenotype called "short tail," which is inherited as a dominant trait. Some individuals display a recessive trait called "dilute," which affects coat color. Which of these traits would be easier to eliminate from the population by selective breeding? Why?

The "short tail" dominant allele is easier to eliminate by selective breeding.

Explanation:

The only way for a recessive allele to be expressed (be visible) is when it appears as recessive homozygotic. These means the organisms need to have 2 copies of the gene. Selective breeding is based on the characteristics that one can see, so if the organism shows the "dilute" phenotype you can keep reproducing this individuals and get rid of the dominant allele.

On the other hand if you have a population with the dominant phenotype, you discard all the ones that have a recessive trait and you breed the dominant phenotype you could still get individuals with the recessive phenotype and individuals that express the dominant phenotype but are heterozygous.