alpha cut means it is the cut off value . the number having member ship value more than or equal to in the given fuzzy set will be considered as present in the result . now in the set OLD person with age 40,50,60,70,80 are having membership values more than or equal to 0.4 so they are considered as member with value 1 and 20 30 with value less than cut off are not member hence they are shown with value 0 so
ans is D ( P.S. it is one form of showing the result another may be {40,50,60,70,80} but it is not in choice )