The contractile unit in the muscle cell is called the
a. z-line
b. myofilament
c. sarcomere
d. myosin

c. Sarcomere

Explanation:

Sarcomere is one of the basic components of striated muscle that allows muscle contraction. That is, it is the contractile unit in the muscle cell.

The sarcomere is made up of a protein complex, the most important of which are actin and myosin. In the presence of Ca2 +, the ends of the myosin bind to nearby actin molecules and fold with great velocity. The actin filament then moves to the center of the sarcomere, triggering the approach of the two Z lines. This causes the sarcomere to shrink and, to a large extent, lead to contraction of the entire muscle.

genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).