A cross in which an organism showing a dominant phenotype (genotype unknown) is crossed with the recessive parent in order to know its genotype is called as
(a) Monohybrid cross
(b) Back cross
(c) Test cross
(d) Dihybrid cross

Hint: In case of normal dominance, a homozygous dominant plant, and a heterozygous dominant plant, both will show the same phenotype which is the dominant phenotype. The genotype of such individuals cannot be determined just by looking at their phenotype and thus a cross is used in which the sibling is crossed with their recessive parent.

Complete Answer:
A test cross is the name of the cross which is used to determine the genotype of a dominant offspring by crossing it with the recessive parent.
If the offspring is homozygous, then all the offspring produced by the cross will show a dominant genotype as they all will be having a heterozygous dominant genotype.

But if the offspring is heterozygous, half the offspring produced from this cross will be recessive as they will have a homozygous recessive genotype.
By observing the phenotype of the offsprings produced from the test cross, we can determine the genotype of the dominant individual.

Additional information: Let us look at the other types of crosses mentioned in the options.
Monohybrid cross: A monohybrid cross-refers to any such cross in which the inheritance pattern of only one character is studied. For example, the cross between pea plants to study the inheritance of tallness.
Back cross: Back cross-refers to any cross in which the offspring from the F1 generation is crossed with any one of its parents be it the dominant or recessive parent.
Dihybrid cross: A dihybrid cross is when the inheritance pattern of two characters such as seed shape along with its color is studied together.
So, the correct option is ‘(c) Test cross’.

Note:
- In incomplete dominance, the dominant allele is not able to fully dominate the recessive allele and thus the heterozygous individual shows a phenotype that is intermediate of the two genotypes.
- In case of incomplete dominance there is no need for a test cross because the phenotype of the homozygous dominant and heterozygous dominant is different and can be differentiated.
- The inheritance pattern of flower color in dog flower plants shows incomplete dominance.