Which one of the following is not explicitly stated in the Constitution of India but followed as convention?

(a) The Finance Minister is to be a Member of the Lower House
(b) Th Prime Minister has to resign if he loses majority in the Lower House
(c) All the parts of India are to be represented in the council of Ministers
(d) In the event of both the President and the vice- President emitting office simultaneously before the end of their tenure the speaker of the lower house of the Parliament will officiate as the President.