Which of the following exercises is most suitable for improving flexibility?

Static stretching is widely recognized as the most effective exercise for improving flexibility. This type of stretching involves holding a position that elongates a particular muscle or muscle group for an extended period, usually between 15 to 60 seconds. This method allows the muscle fibers to relax and lengthen, which can enhance range of motion and overall flexibility over time.

In contrast, other exercises like the plank and squats primarily focus on strength training and stability rather than flexibility. The plank engages the core muscles and helps to promote stability and endurance, while squats target lower body strength, particularly in the quadriceps, hamstrings, and glutes.

Cycling, although beneficial for cardiovascular fitness and lower-body endurance, does not significantly address flexibility, as it mainly involves repetitive movements without the sustained stretch that static stretching provides.

Thus, static stretching stands out as the most suitable option specifically for improving flexibility, making it the correct choice in this context.