Right option is (a) Buttock plane is at a distance of 40 unit from fuselage centerline
To explain: Butt-planes or buttock planes are defined as vertically oriented cuts which are used for fuselage lofting verification. It cuts from the horizontal intersection of the A/C with vertical planes. These planes are defined by their distance from centerline. Hence, butt-plane 40 can be interpreted as butt plane is at distance of 40 unit from aircraft centerline.