Which ion binds with troponin during muscle contraction?

Which of the following is not a contractile protein?

The contractile unit of a muscle fiber is known as:

The time period between the beginning of the electrical response and the peak of tension recorded is known as:

Consider the following four statements $(a-d)$ and select the option which includes all the incorrect ones:
$(a)$ Skeletal muscle fibre is a syncytium as the sarcoplasm contains many nuclei.
$(b)$ In the centre of each $I$-band is an elastic fibre called $H$-line which bisects it.
$(c)$ In the resting state,a subunit of troponin masks the active binding sites for myosin on the actin filaments.
$(d)$ The contraction of a muscle fibre takes place by the sliding of the thick filaments over the thin filaments.

Action potential in the sarcolemma of muscles causes the release of: