One of the 'Organisation' or 'Organisations'

Question

I am writing an essay and stuck with the following sentence.

One of the non-profit organisation that we worked with...

Should organisation be plural or singular?

Answer 1

Organisation must be plural here.

This is because when you say

one of ___

you mean

one of many.

At least it would mean one of two.

So out of the many organisations, you're referring to one.

And the usage is therefore "one of the non-profit organisations"