Which one of the following acts as a second messenger?

Many extracellular ligands act by increasing the intracellular concentrations of second messengers such as cyclic adenosine-3',5'-monophosphate (cAMP), calcium ion, or the phosphoinositides. Calcium is an impoant messenger in all cells and can regulate diverse responses including gene expression, contraction, secretion, metabolism, and electrical activity. In keeping with its role as a signal, the basal Ca2+ level in cells is maintained in the 100 n range by membrane Ca2+ pumps that extrude Ca2+ to the extracellular space and a sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA) in the membrane of the endoplasmic reticulum (ER) that accumulates Ca2+ into its storage site in the ER/SR. Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 2. Overview of Cellular Physiology in Medical Physiology. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong's Review of Medical Physiology, 24e.