A cross between two organisms, one of which has an unknown genotype and the other with a homozygous recessive trait is called a test cross. This cross was first performed by Gregor Mendel and is used to detect the unknown genotype of a particular organism. It is a valuable tool in genetics. Hence a cross of pea plants in which one of the parents has a homozygous recessive genotype is called a test cross. Thus the correct answer is option C.