The conclusion is : Therefore the envelopes, and thus these mailings, are
completely recyclable.
Why does the president say so? Well, he says that the paper and stationary are made from recycled paper, that the envelopes don't contain anything other than letters (as in there is no plastic material for example - the envelope carries letters and nothing else) and that even the window of the envelope is made from recyclable material. (the window is that small transparent part of the envelope in which you can see the contents or the name and address of the person to whom the envelope and its contents are sent).
Now, the president is assuming that the fact that an item is made from recyclable material means that the item is recyclable. Please remember the conclusion again because the conclusion says that the entire envelope (envelope plus letter plus window) is completely recyclable. Focus on the word completely.
(A) All the paper used by the organization for purposes other than mailings is recycled.
> Not bothered with materials used in other purposes. The president is talking about mailings.
(B) The mailings from the organization’s national headquarters always use envelopes that have windows.
> Okay but what does that have to with the president's argument that the envelope is completely recyclable? Even if that they don't use windows, how does that impact the conclusion? The conclusion is about recycling not about whether windows are used on envelopes.
(C) The envelope windows made from recycled material are recyclable.
> This doesn't seem like the right answer but try negating this. It will read - The envelope windows made from recycled material are not recyclable. Then the argument falls apart right? The argument says that the complete thing is recyclable but if one part is not then can we say that it is completely recyclable?
(D) The envelopes and stationery used in the organization’s mailings are always recycled.
> If you negate this, it will read - The envelopes and stationery used in the organization’s mailings are not always recycled. Okay, so some times they recycle the material. The conclusion is about can the material be recycled and not about is the material recycled. In the latter, you are assuming that the material is capable of being recycled and whereas in the former, you are questioning the material is capable of being recycled - what the argument is about.
(E) The organization sends mailings only from its national headquarters.
> The location is not important. Reject this.
C is your answer.