Many elements are found in living organisms either free or in the form of compounds. One of the following is not found in living organisms.

(a) Silicon

(b) Magnesium

(c) Iron

(d) Sodium

(a) Silicon is not found freely in nature, but it does occur as oxides and silicates, whereas magnesium, iron and sodium are present in living organisms as ions. Silicon is essential to plant life but is often found in minute quantities in human body and its function is still unknown.
Magnesium is an abundant element. It is essential for a number of enzymes and their action, particularly those utilising ATP.
Iron is an important constituent of haemoglobin and plays a vital role by taking part in O2 transport, and electron transport chain. 
Sodium plays a vital role in animals by regulating nerve impulse transmission and altering the membrane permeability. It also has indispensable role in osmoregulation.
Page 143, TABLE 9.1, XI NCERT