The assumption/assumptions of the argument is /are which of the following?
I. Hydro electric power is a renewable source of energy.
II. Hydro electric power is comparatively cheaper.

Statement 1 is a fact stated in the premise. The author says that "it has become vital that such renewable sources of energy are tapped." while referring to hydroelectric power. As the information is given in the premise, it is not an assumption but an explicitly stated fact.

Also the author states that the price of the oil has been increasing and that's why we have to tap hydroelectric power. The author has assumed that the hydroelectric power is cheap and renewable. Hence, statement 2 is an assumption made by the author.