Question

Which is true of muscle contraction
A. Sarcolemma becomes permeable to $Ca^{2+}$ ions
B. Sarcolemma becomes permeable $Na^+$ ions
C. Sarcolemma becomes non-permeable to $Na^+$ ions
D. Concentration of $Ca^{2+}$ ions is reduced in myoplasm

Answer 1

Muscle contraction is the activation of tension-generating sites within muscle fibers. The sarcolemma is also called the myolemma. It is the cell membrane of a striated muscle fiber cell. Sarcolemma consists of a lipid bilayer and a thin outer coat of polysaccharide material (glycocalyx) that connects the basement membrane.

Complete answer:
The $Ca^{2+}$ influx causes synaptic vesicles containing the neurotransmitter acetylcholine to fuse with the plasma membrane, releasing acetylcholine into the synaptic cleft between the motor neuron terminal and the neuromuscular junction of the skeletal muscle fiber. Acetylcholine diffuses across the synapse and binds to and activates nicotinic acetylcholine receptors on the neuromuscular junction.
Activation of the nicotinic receptor opens its intrinsic sodium/potassium channel, causing sodium to rush in and potassium to trickle out. As a result, the sarcolemma reverses polarity and its voltage quickly jumps from the resting membrane potential of -90mV to as high as +75mV as sodium enters. The membrane potential then becomes hyperpolarized when potassium exits and is then adjusted back to the resting membrane potential. This rapid fluctuation is called the end-plate potential.
So, the answer is B, i.e., Sarcolemma becomes permeable $Na^+$ ions.

Note: $Ca^{2+}$ ions play an important role in muscle contraction by creating interactions between the proteins, myosin, and actin. The $Ca^{2+}$ ions bind to the C component of the actin filament, which exposes the binding site for the myosin head to bind to in order to stimulate a muscle contraction.