The most important factor in decreasing the intracellular concentration of calcium ion after contraction is

Active transport of calcium into the sarcoplasmic reticulum.

Explanation:

The sarcoplasmic reticulum (SR) is a membrane-bound structure found within muscle cells that is similar to the endoplasmic reticulum in other cells. The main function of the SR is to store calcium ions (Ca2+) In response to an action potential traveling along the t tubule, voltage-sensitive proteins change shape, opening calcium channels in the adjacent terminal cisterns of the sarcoplasmic reticulum.