How to use commas and “and” in a list of three items where the first two items are followed by a parenthetical remark “(both…)”?

Let's assume you adhere to the Oxford-comma style and write American English. Then, beyond any reasonable doubt, the sentence

A. Mary bought fish, meat (which smelled rotten), and bread.

is perfect.

Now consider the sentences

B. Mary bought fish, meat (both of which smelled rotten), and bread.

C. Mary bought fish and meat (both of which smelled rotten), and bread.

Where to put the commas in these two sentences and why? Do you prefer B or C (and why)?

The sentences B and C present a particular difficulty to me, since the scope of the parenthetical remark extends not only over the second item (as in A, where we have a simple list of three items) but also over the first item of the list (so what we have is probably not a simple list of three items).

It is not a matter of preference. B is incorrect.

As you say, when you describe the fish and meat with the same parenthetical clause you turn them into one unit, syntactically speaking. If you zoom out a little,1 you can see that Mary bought two things: Thing A (which smelled rotten) and Thing B. The fact that Thing A actually consists of A1 and A2 does not change how the overall sentence works.

In fact, because the list now only consists of two (main) items, the comma after the close-parenthesis is incorrect and you are left with:

C. Mary bought fish and meat (both of which smelled rotten) and bread.

1I had a calculus teacher once who explained that sometimes you have to "blur your vision" and focus only on the "outside" function which has some argument theta and ignore the fact that theta is a complicated function in its own right.

• Although a list of two items is usually written as “A1 and A2” and sounds perfect this way, I do not think it is incorrect to write “A1, A2” (even though it sounds strange), isn't it?
– user142975
Commented Sep 10, 2021 at 23:30
• It is, in fact, incorrect to write a two-item list that way (although newspaper editors do it all the time headlines). Commented Sep 11, 2021 at 0:17
• Headlines are not standard English, they are headlinese, and follow entirely different rules of grammar. For example, they use the infinitive to express the future, the simple present to express the past, and they rarely bother with the copula at all. Commented Sep 11, 2021 at 0:51