Which of the following is considered the active form of calcium?

In blood viually all of calcium is in the plasma, which has a mean normal calcium concentration of approximately 9.5mg/dl. calcium exists in 3 chemical states in plasma, of which 50% is Ionised , 40% is bound to proteins, 10% is complexed with small anions. The free or ionised calcium is the biologically active form . Its concentration in plasma is tightly regulated by vitamin D and parathyroid hormone. Teitz textbook of clinical biochemistry , 6th edition,section 5, page number 1736.