A testcross is a mating between an individual with a dominant phenotype (but unknown genotype) and

Homozygous recessive individual

Explanation:

A testcross is a cross performed to test the genotype of an organism. The test cross is performed when the nature of the phenotype that is if the phenotype is dominant or recessive is known.

We know that a recessive trait is the result of the homozygous condition or phenotype is expressed when the genotype has two recessive alleles. But the genotype of the dominant trait could be determined when the genotype has either both the dominant alleles (homozygous dominant) or the either one of them is dominant allele (heterozygous condition).

So to determine if the dominant phenotype is determined by homozygous genotype or the heterozygous genotype, testcross is performed between the homozygous recessive genotype and the unknown dominant phenotype organism.

Thus, Homozygous recessive individual is the correct answer.

stratosphere: as the altitude increases, the air temperature increases. the stratosphere has a layer of ozone, called the ozone layer. this layer absorbs most of the ultraviolet radiation from sunlight. this results in the stratosphere being warmer.