A farmer bought a sheep who is reported to be pure for her dominant fluffy fur. He is suspicious that she is actually a hybrid. How could the farmer perform a
testcross?

by crossing the sheep with a homo-zygous recessive sheep for “fluff” alleles

Explanation:

In genetics, the term homo-zygous refers to an individual that carries two copies of the same gene variant or 'allele' for a given gene. An individual that is homo-zygous dominant carries two copies of the same dominant allele. A dominant allele is a gene variant that can mask the expression of a recessive allele in heterozygous individuals (i.e., individuals that carry one copy of the dominant allele and one copy of the recessive allele). In a test cross, an individual that is suspected to be heterozygous for a given gene is crossed with a homo-zygous recessive individual in order to examine the phenotypes of the offspring. In this case, if the sheep that is suspected to be heterozygous (i.e., hybrid) is really heterozygous for the fur gene (genotype Ff), then the phenotypic ratio of the testcross progeny (F1) would be 1:1, i.e. 50% of the F1 individuals would have the 'dominant fluffy fur' phenotype (genotype Ff) and 50% of the F1 individuals would have the 'recessive non-fluffy fur' phenotype (genotype ff). Conversely, if the sheep that is suspected to be heterozygous is homo-zygous dominant for the fur gene (genotype FF), then 100% of the F1 individuals would have the 'dominant fluffy fur' phenotype (genotype Ff).

the correct answer is b