B
Step-by-step explanation:
First prime factorize each number; this means breaking up the number into the product of only prime numbers.
24 = ![2^3*3](/tpl/images/0598/6523/bb2ff.png)
36 = ![2^2*3^2](/tpl/images/0598/6523/4ff4b.png)
72 = ![2^3*3^2](/tpl/images/0598/6523/b0453.png)
Looking at these three numbers, we see that the biggest number they share is of the form:
, where is just 12.
Thus, the answer is B.
Hope this helps!