During the contraction of a vertebrate skeletal muscle fiber, calcium ions
a. break cross-bridges by acting as a cofactor in the hydrolysis of ATP.
b. spread action potentials through the T tubules.
c. re-establish the polarization of the plasma membrane following an action potential.
d. transmit action potentials from the motor neuron to the muscle fiber.
e. bind with troponin, changing its shape so that the myosin-binding sites on actin are exposed.

e.

Explanation:

Muscle contraction is defined as the activation of tension-generating sites within muscle fibers.

Contraction of a vertebrate skeletal muscle fiber is initiated when calcium ion binds to the troponin, a complex fibre protein. Binding of calcium allows the exposure of active-binding sites present on the actin. The uncovereed actin-biding sites allows the the high-energy myosin head to bridge the gap forming cross bridge. the cross-bridge than support release of ADP and synthesis of ATP for contraction.

Hence, the correct answer is e.

correct answer is a. and d.