The answer to the question is that it is
Myofibrils are the building blocks or contractile unit of each muscle fiber. The tiniest functional unit of skeletal muscle, the sarcomere, is found in the myofibrils and is made up of protein filaments: actin (thin) and myosin (thick).
The answer is the second and third answers. Kingdom Protista is a polyphyletic taxon, meaning they did not evolve from one common ancestor, unlike what you see from a monophyletic taxon. Because they come from multiple ancestors, they share more similarities with other organisms from other kingdoms, rather than themselves.
