Folder is to document as envelope is to what
The correct answer is option C.
A folder is to document as the envelope is to Letters. So, the correct option is letters.
A folder is a sheet of cardboard which is light in weight and made for holding papers just as we keep papers in a file.
An envelope is a common packaging item. It is usually made of flat material which is very thin. It is designed to hold objects which are flat in shapes such as a letter or a card.
Folders and envelopes are made of a sheet of paper cut to one of three shapes.
A document usually tends to be placed within a folder. Thus we need to find a term for something that is placed within an envelope. Since a letter usually tends to be placed within an envelope, so, the correct answer is a letter.