Answer: The correct option is C.
Step-by-step explanation:
In the given figure we can say that the triangle ABC is an isosceles triangle, because two sides AB and AC of the triangle are same.
In a isosceles triangle the angles corresponding to the same sides are equal. Therefore,

It is noticed that angle ABC and exterior angle B lies on a straight line it means they are supplementary angles and their sum is 180 degree.

Combine like terms.

Subtract both sides by 20.

Divide both sides by 8.

The value of x is 20. Therefore the correct option is C.