Hi! Like you wrote down, a supplementary angle is when the two angles equal 180 degrees when added together. You can inference that angle B will be 142 degrees (A+B=180, 38+B=180, B=142). Therefore, it cannot be anything less or more than 142 if you want it to be a supplementary angle. The answer is D, which shows that the answer is 142 and nothing more or less.
