Ans.
In vertebrates, genes responsible for the growth and development of limbs in developing embryos are known as Hox genes (homeotic-box genes). Along with limb development, these genes include a group of genes that control the head-tail axis and body plan of developing embryo.
A single mutation in a Hox genes can lead to formation of mutated Hox protein and turn off the growth of legs in an organism.
Thus, the correct answer is 'option B). a Hox gene.'