The correct answer is B. Changing the muscle length
Step-by-step explanation:
Isometric exercise is a type of exercise in which muscle is strengthed through contractions but without the movement of joints or the muscle changing its leg. This occurs because these exercises imply keeping a specific position that represents physical effort but no movement. For example, planks in which you have to lift your own weigh by putting all your weight on your elbows, toes, and forearms is an isometric exercise because it involves contracting your muscles and making an effort but there is no movement and therefore the length of muscles do not change.