Which set of values could be the side lengths of a 30-60-90 triangle?

First of all, with 30-60-90 triangles, we know this ratio applies:

Shortest side: middle length side: hypotenuse
           1          :             √3              :         2

Therefore, with this is mind, the only correct answer out of a,b,c and d is b. 

Finally, to further prove b is correct, let us check with the ratio:

Ratio:           1          :             √3              :         2
Option b      6         :             6√3           :         12

So as you can see, Option b matches with the 30-60-90 ratio.
(Each ratio number can be divided by 6 to get the 30-60-90 ratio.)

Hope this helps!