In which compartment of the body is the majority of water stored?

Intracellular fluid (ICF) compartment

Explanation:

Intracellular fluid compartment refers to the intracellular fluid or cytosol the fluid contained inside the cells. About two thirds of the total body water is located in the cells, mostly in the cytosol. The intracellular compartment contains on average about 8 litres of fluid, with moderate quantities of magnesium and sulphate ions. The cytosol together with organelles forms the cytoplasm.

Intracellular compartment is seprated from the extracellular compartment via semi-permeable membrane.