Final answer:
During the lowering phase of a push-up, the sustained position of the knees is flexion.
Step-by-step explanation:
During the lowering phase of a push-up when bending at the elbows, the knees are in flexion.
Flexion is a movement that decreases the angle between two body parts. In this case, as you lower your body towards the ground, your knees bend and the angle between your thigh and lower leg decreases.
Therefore, the correct answer to the question is a) Flexion.