Final answer:
The ion with the lowest cytosolic concentration among the options given is the calcium ion (Ca2+), due to the action of ion pumps that maintain its low concentration in the cytosol for signaling purposes.
Step-by-step explanation:
The student's question asks which ion has the lowest cytosolic concentration among H+, K+, Mg2+, and Ca2+. Based on the information provided regarding the functions and regulation of calcium ions (Ca2+) within a cell, it is clear that calcium ions have a very low free concentration in the cytosol. This low concentration is maintained by ion pumps in the plasma membrane that continuously use ATP to transport Ca2+ out of the cytosol. Calcium is stored in cytoplasmic vesicles like the endoplasmic reticulum or accessed from outside the cell, and its influx into the cytosol is usually a tightly regulated process that occurs during signaling events.