Final answer:
The supplementary angle to a given angle A is found using the expression 180 - A. Hence, the correct option is A. 180 - a.
Step-by-step explanation:
The question is about finding the supplementary angle for a given angle. In this context, the supplementary angle to angle A would be given by the expression 180 degrees minus the measure of angle A. Therefore, the correct option from the list provided would be A. 180 - a, since the sum of angle A and its supplementary angle must equal 180 degrees.