Final answer:
Removing the saddle during a cycling class improves participants' posture and forces more active engagement of the core muscles, which is important for maintaining balance and reducing back strain.
Step-by-step explanation:
The question concerns how removing the saddle during a cycling class affects participants' posture and muscle engagement. Removing the saddle requires participants to maintain their balance and posture using their core muscles, which increases the engagement of these muscles and can improve overall posture. In contrast to options B, C, and D, the correct answer is A: It improves posture and engages core muscles more effectively. Good posture is characterized by the upper body's center of gravity being directly above the pivot point in the hips, which reduces the need for muscle action to maintain balance and prevent back strain. Without the saddle, participants must engage their core muscles to a greater extent to maintain this alignment and avoid the need for excessive force by the back muscles to counteract any torque produced by the upper body's weight.