UPSC NDA I 2021 Mathematics Solved Paper

Concept:
The projection of any point on (a,b,c) on x-axis is (a,0,0),y-axis is (0,b,0) and z-axis is (0,0,c).
Calculation:
The projection of A(1,7,−5) on y-axis is (0,7,0) and projection of B(−3,4,−2) on y-axis is(0,4,0).
Let C=(0,7,0),D=(0,4,0) and O=(0,0,0) is the origin
⇒OC=7‌ and ‌OD=4
Let the projection of line segment AB is m=OD−OC.
⇒m=OD−OC=7−4=3
Hence, correct option is 3 .