Answer: b. Muscles
A metazoa is a division of animal kingdom that includes all animals expect protozoa and sponges. Metazoa division includes multicelluar animals, which exhibit highly differentiated cells. They have muscular and nerve system and well coordinated tissues and organs. Locomotion requires a coordination activity of muscular, skeletal and neural system. It is mainly achieved by contraction and relaxation of muscles that occurs due to specialized muscle proteins named as actin and myosin which receives signals from nerves.
Hence, locomotion in metazoa is due to contraction of muscles.