Here it says an electron in which of the following orbital is the most likely to be found along the X&Y axis. So they said along the axis or on the axis. So there's only two possibilities and those are D sub X2 -, y2, or DZ squared.
We can see from the designations here we're talking about along the X&Y axis. The first one is the one that contains those two variables of X&Y. So here that would mean that answer or option D would have to be our correct answer.
The first three do not work because they are in between the axes and then DZ squared. Here we're talking about the one that's predominantly on X&Y and that have to be option D over option East.