Final answer:
Without additional context, if angle DCA and angle ABC form a linear pair, angle ABC would measure 160 degrees, as angles in a linear pair sum to 180 degrees.
Step-by-step explanation:
To determine the measure of angle ABC, we need additional information about the shape or the context in which angle DCA and angle ABC are related. However, if angles DCA and ABC are parts of a linear pair or if angle ABC is the exterior angle of triangle ADC, then we could infer that angle ABC would be the supplement of angle DCA. Assuming DCA and ABC form a linear pair, which sum to 180 degrees, the measure of angle ABC can be found by subtracting the measure of angle DCA from 180 degrees.
Since angle DCA measures 20 degrees, angle ABC would measure:
180 degrees - 20 degrees = 160 degrees.
Therefore, the correct answer is:
a) 160 degrees