Answer is : (c) Both the parents would be normal
Colourblindness is a X-linked recessive disease, i.e. an heterozygous mother does not show the disease and is a carrier. But a father cannot be a carrier of the disease as it has single X-chromosome.
In the given question, the son is colourblind which means it had inherited Xc from the mother. But another son is normal. This shows that the mother is heterozygous for the disease.
The expected cross for the question will be
This shows that both the parents would be normal if they have one colourblind son and one normal son.