It Is Believed That a Nucleotide Change from T to C

It is believed that a nucleotide change from T to C in the gene encoding a green pigment protein is responsible for certain cases of color blindness. In order to test that a mutation in this gene is responsible for color blindness you would want to

A) run gel electrophoresis to ensure it is the right gene.
B) carry out knockout analysis to observe what happens when the gene is not expressed.
C) perform plasmid transformation of the DNA into a vector and observe changes in bacteria.
D) perform a Southern blot to account for the C to T change in the sequence.