Expression : XYZXYZxYZ, XYZXYzXYZ, XYZXyZXYZ, XYZxYZXYZ, XYzXYZXYZ, _______________. The pattern followed is that in each term there are 3 combinations of 'XYZ' and in each term, one letter is small and its position is changing in decreasing order. Thus, in the missing term, the second letter will be small = XyZXYZXYZ