Final answer:
Mackenzie and Mary are practicing PNF stretching by aiding each other, which is a method involving active and passive roles to enhance flexibility.
Step-by-step explanation:
Mackenzie and Mary are engaging in PNF stretching, short for Proprioceptive Neuromuscular Facilitation. This type of stretching involves both an active and a passive partner, where one provides resistance while the other stretches, aiding in improving flexibility and strength. Since they are aiding each other in stretching, the correct answer is b. PNF.