Concept: The question tests on the basics of geometry

Solution:
 

A new og 2021 question

We know that the external angle is obtained as a sum of two opposite internal angles in a triangle.

=> z = x + y

St(1):- x = 50

We do not know the value of y to figure out the value of z. (Insufficient)

St(2) :- y = 35

We do not know the value of x to figure out the value of z. (Insufficient)

Combining both we have

x=50 and y =35
Thus z = x + y
= 50 + 35 (Sufficient) Option (c)