Which of the following ions is necessary for the contraction of muscles and nerve impulse transmission?

Rahul exercises regularly by visiting a gymnasium. Of late,he is gaining weight. What could be the reason? Choose the correct answer and elaborate.
$A$. Rahul has gained weight due to accumulation of fats in the body.
$B$. Rahul has gained weight due to increased muscle mass and loss of fat.
$C$. Rahul has gained weight because his muscle shape has improved.
$D$. Rahul has gained weight because he is accumulating water in the body.

Based on their location in the human body or animals,the types of muscles are:
$I.$ Skeletal
$II.$ Visceral
$III.$ Cardiac
$IV.$ Non-visceral
The correct option is:

Cardiac muscle fibers differ from skeletal muscle fibers in that they are -

$ATP$ binding site is located on the

Given below are two statements $:-$
Statement-$I$ :- Each organised skeletal muscle in human body is made of a number of fascicles,held together by a common connective tissue layer called fascia.
Statement-$II$ :- The portion of the myofibril between two successive '$Z$' lines is considered as the functional unit of muscle contraction,known as a sarcomere.
In the light of the above statements,choose the correct answer from the options given below $:-$