State any one reason why the President is elected indirectly.

Because the President is only the NOMINAL head of the State, while real executive power rests with the Prime Minister and the Council of Ministers. A direct election by the whole nation would give him a popular mandate out of keeping with that limited position and might set him against the real executive. (Equally acceptable: a direct election in a country of India’s size would cost enormous time, money and energy.)

Only ONE reason is asked, so give one and give it well rather than listing several. The nominal-head argument is the constitutional reason and the stronger of the two; the cost-and-size argument is the practical one. Either earns the mark. The Electoral College is worth knowing for the fuller question: the elected members of both Houses of Parliament and of the State Legislative Assemblies — nominated members do not vote.