Thanks! I was hoping the answer choices would allow for a "shortcut" to the answer, but that's not the case here. I would guess that TBR's reference to "averaging the solutions" simply means (total moles) / (total volume), as described by Asteri, which is probably the most straightforward way of solving this question.
However, here's a different approach that's a little more conceptual and easier on the math. Maybe it'll be faster for you, maybe not.
The first solution is 0.1 M KCl, which is effectively 0.1 M Cl-
The second solution is 0.2 M MgCl2, which is effectively 0.4 M Cl-
The molarity of Cl- of the mixed solution is the average of 0.1 and 0.4 M weighted by their respective volumes (50 and 25 mL). Notice the 2:1 ratio of these volumes; this is 2 parts 0.1 M and 1 part 0.4 M. This means that the answer will be closer to 0.1 M than it will be to 0.4 M (which unfortunately cancels out only 0.25 M, I was hoping for more!)
If you simply calculate the average, [2(0.1)+(0.4)]/3 = 0.2
If you're into fractions, the average is one third the way between 0.1 and 0.4 M, or 0.2 M