The right answer is the third option:
10^-3 L / 1 mL
That conversion factor may be derived from 1 L = 1000 mL
=> 1 L / 1000 mL = 1
=> 10 ^ - 3 L / 1 mL = 1
So, you can mutilply by the conversion factor 1 * 10^ -3L / 1 mL because it the equivalent to 1.
The other options are not valid. Let's see why:
1 KL = 1000 L => 1KL / 1000 L = 1
=> 10 ^ - 3 KL / 1 L, which is contrary to the first option.
In that same way you can find the righ conversion factors that relate L with cL, and L with d and you will discard the other options.
