Calcium is important in skeletal muscle contraction because it

The correct option is C binds to troponin to remove the masking of active sites on actin for myosin
Sliding filament theory of muscle contraction was proposed to explain the mechanism of muscle contraction. It states that the thin actin filaments slide over the thick myosin filaments resulting in the contraction of muscles.

The thin filaments in muscle fibres are made of F-actin (filamentous actin), tropomyosin, and troponin proteins. F-actin and tropomyosin are long molecules wound around each other. Troponin is a much smaller protein molecule and is located at regular intervals on the thin filament.

Each myosin (thick) filament is composed of monomeric proteins, meromyosin. Each meromyosin has two important parts, a globular head with a short arm and a tail. The globular head is an active ATPase enzyme and has binding sites for ATP and active sites for actin. The myosin head binds and hydrolyses ATP into ADP and phosphate.

Fig: Actin and myosin filaments

During a muscle contraction, when an action potential arrives, calcium ions are released from the sarcoplasmic reticulum into the sarcoplasm. They bind with the troponin
molecules, causing the strands of tropomyosin to shift, thereby exposing the myosin-binding sites on the thin filaments.

As soon as the actin-binding sites are uncovered, the myosin head binds the actin filament, forming a cross bridge. Using the energy of ATP hydrolysis, it pulls the actin filament towards the centre of the sarcomere. Next, the myosin head releases the bound ADP and phosphate. Then, it binds a fresh ATP molecule and the cycle continues, as long as there are abundant calcium ions in the sarcoplasm.