How to find the the angle measurement ?

That is a 180 degree angle (the straight line) and one angle is 78, so you subtract 78 from 180
So the angle would be 102 degrees

When finding an angle, it's always important to know a rule.
The rule in this case is 'Angles on a straight line sum to 180'

So in this case there is a 78 degree angle. 
180 - 78 = 102 degrees for the remaining angle.

Iam thinking the answer is a.) knew