Why is a testcross necessary to determine genotype when an organism shows a dominant trait?
a. phenotype has nothing to do with genotype.
b. an organism with a dominant trait will always have two dominant alleles.
c. an organism only needs one dominant allele to show the dominant trait.
d. testcrosses aren't required when dominant traits are present

option A, an organism only needs one dominant allele to show the dominant trait.

Explanation:

A test cross between two individuals helps to determine the genotype of off springs and thus decide their phenotypic characteristics.  

Since one single dominant allele is enough to express the dominant trait, it becomes essential to know the exact genotype of organism showing dominant trait

For example –  

Let red colour be a dominant trait of flower represented by “R”

And white colour be a recessive trait of flower represented by “w”

If a homozygous red and white flower are crossed the following offspring would form –

RR

wRwRw

wRwRw

Now , since red is dominant over white all the offs pings flower would be of red color. Now in case if exact genotype is not determined , then one would assume that all red offspring have phenotype “RR” and this will create problem in predicting output of future crosses carried out between offspring.  

Thus, knowing the genotype is extremely important to determine both the genotype and phenotype of future generations.