The most common methods of stress testing include
a. arm ergometer and treadmill.
b. bicycle ergometer and stair-stepper.
c. treadmill and bicycle ergometer.
d. stair-stepper and arm ergometer

c. treadmill and bicycle ergometer.

Explanation:

A stress test can be defined as a kind of exercise which is performed to record how the heart works during the physical activity. As the exercise causes the pumping of heart harder and faster. It is a test which gives information about the flow of blood from the heart, blood pressure and breathing are continuously monitored.  

A stress test is usually performed by walking on a treadmill or riding on a stationary bike or bicycle ergometer. As continuous walking and riding is likely to increase the functional rate of heart.