Stained glass windows of cathedrals

C too, but I think the second sentence is a discription rather than an explaination.

I would go with B. The author is definitely citing example to make her point. What is the OA please?

I would go with B too.

She cites the example of modern magazines to illustrate her conclusion that stained glass windows of cathedrals were perceived as chronicles of the ordinary lives of townspeople.

I will go with (A), its a decent description of the passage & its better than the alternatives (although the use of the word 'foreign' made me doubt a little bit at first). The author writes "like a modern magazine" to help the reader relate to what may be an unfamiliar topic (ancient stained glass window art).

(B) A modern magazine is not an example of a stained glass window. After the "magazine" comparison, the author went on to describe the various functions of the stained glass "magazine". If she was trying to cite examples, we should see something like "for instance, in church X, this window told the story of..."
(C) The author stated her opinion, but did NOT explain WHY she thinks glass stained windows are used for storytelling, she merely made a comparison.
(D) I don't think we can tell whether her statements are fact or opinion. If glass windows were indeed used for storytelling, then it is NOT merely opinion, it would be an established fact; but we can't tell either way.
(E) This is a trick answer. "Reducing art to function" is what the makers of the glass windows might have done, NOT what the author did.

I thought it would be C, but depending upon the above explanation, I think A is more appropriate.