'...WHY HE HAD TO MARRY AGAIN...' IS GRAMMATICAL NAME AND IS FUNCTION

IS FUNCTION

dependent clause

Function depends on how it’s used in a whole sentence.

Surbodinate clause

It is not a complete sentence

Noun Phrase

It express the word marry

What is the grammatical name of why he had to marry again

Noun phase

That's incorrect, the grammatical name of "why he had to marry again" is a subordinate clause.

This is a noun phrase

The answer is either noun clause or independent clause