Q:

What does the particle at the end of なぜと申しますと mean?

I've always known that と may be used as a particle that is required for the formation of a conditional phrase or to quote someone's speech. So, what is the function of the last particle in the following sentence?

なぜと申しますと

A:

「なぜと[申]{もう}しますと」

In this case, 「と」at the end is a conjunctive particle that serves as a preface to the statement that follows.
Thus, the only statement that can logically follow 「なぜと[申]{もう}しますと」 is one that explains the reason for whatever is being discussed.
The 「と」 in 「なぜと」, by the way, is quotative in case anyone is wondering.  So we have two different kinds of 「と」 used in such a short phrase.
Usually, the entire sentence will look like:

「なぜと[申]{もう}しますと、～～～～からです。」 =
"To tell you the reason, it is/was because ~~~~."

(This might sound off-topic, but in real life, saying 「なぜかと申しますと」 is definitely more common than saying 「なぜと申しますと」.  That, however, makes no difference in the function of 「と」 at the end.)

