If a single mutation turns off the growth of some pairs of legs within an organism, what's most likely affected?
a. a duplicated gene
b. a hox gene
c. a substituted gene
d. a deleted gene

Answer is A. A Hox gene

Hox genes are a series of genes that controls the differentiation of cells and tissues in an embryo by turn on and off other genes. If Hox genes express incorrectly, a major changes in the morphology of the individual can be seen.

Let’s understand this with an example: The Ultrabithorax, a Hox gene determines the Third thoracic segment in Drosophila melanogaster. Third thoracic segment displays a pair of legs and a pair of reduced wings used for balancing. In the mutant Drosophila the functional Ultrabithorax protein is absent. Due to this mutation, the third thoracic segment now expresses the same structures found on the second thoracic segment, which contains a pair of legs and a pair of wings which are fully developed .  

c. live where it’s cold and icy

since there are no options posted i'll give you a few examples.

a dolphin's flipper, bird's wing, cat's leg.