Option "A" is correct.
Step-by-step explanation:
As we know that a line perpendicular to another line always makes right angle (i.e. 90°) with that line, which in this case is equal to the total angle of 6X and 4X.
THUS .:
6X +4X = 90°
=> 10X = 90°
=> X = 90/10 = 9