Given two planes: x−ay−b=0 and cy−z+d=0 Let, l,m,n be the direction ratio of the required line. Since the required line is perpendicular to normal of both the plane, therefore l−am=0 and cm−n=0 ⇒l−am+0.n=0 and 0.l+cm−n=0 ∴
l
a−0
=
m
0+1
=
n
c−0
Hence, d.R of the required line are a,1,c. Hence, options (c) and (d) are rejected. Now, the point (a+b,1,c+d) satisfy the equation of the two given planes. ∴ Option(b) is correct.