Bunuel wrote:

Is the measure of an exterior angle of triangle XYZ equal to 140°?

(1) The measure of angle Y is 60°.

(2) The measure of angle Z is 70°.

If an interior angle is 40 degrees, then the exterior angle adjacent to that interior angle will be 180-40 = 140 degrees.

(1) Exterior angle adjacent to Y = 180-60 = 120 degrees

Now its possible for another interior angle (X or Z) to be 40 degrees, but its not necessary. So we cannot say whether there is an exterior angle of 140 degrees or not.

Not sufficient.

(2) Exterior angle adjacent to Z = 180-10 = 110 degrees

Now its possible for another interior angle (Y or X) to be 40 degrees, but its not necessary. So we cannot say whether there is an exterior angle of 140 degrees or not.

Not sufficient.

Combining the statements,

Two angles are 60 and 70 so third angle must be 50 degrees. None of the three exterior angles is of 140 degrees measure. Sufficient.

Hence

C answer