What kind of clause is the underlined part of the sentence given belo…

The correct answer is Option (3) → Noun clause

Ask: Nikhil asked the ticket checker what? The answer is “when the Rajdhani would reach Patna”, so the clause is the object of “asked”.

Clause	What it does	Example
(1) Adverb clause	an adverb clause of time tells when the action of the main verb happens; this clause is the thing Nikhil asked	Call me when you reach the station.
(2) Adjective clause	describes a noun just before it; it does not describe “the ticket checker”	We often talk about the day when we moved to Bhopal.
(3) Noun clause (answer)	does the work of a noun; it can be the object of a verb and answers the question what?	Riya asked where the post office was.
(4) Principal clause	the main clause; it makes full sense on its own	The children cheered when the rain stopped.