Yes it is a 4 sided polygon known as a quadrilateral
The answer is an envelope. It contains a letter.
Envelope is a noun.
It does
The word is "envelope" (which usually holds only one letter).
it seems to me that the only polygon of your description is a square
A type of polygon is a rhombusial polygon, trysectalnict polygon, and a equilateral polygon.
A triangle (not traingle) is a polygon. It can be a regular polygon but need not be.
on the envelope.
an arrow is or isn't a polygon?
When sending a wedding invitation the response card should go into the response envelope and you should address it back to you on the front of the response envelope with a stamp on the envelope.
This envelope is like the window envelope,but the window opening is uncovered (an uncovered address panel).
The prepositional phrase in the sentence is "on the envelope".