Since S.A. is an unspaced abbreviation, option 1 is correct. I can’t find a specific rule about this stated in the code, but Example 10-45 in Rule 10 illustrates a 2-letter abbreviation with periods (p.e.) in a math expression, and there are no English-letter indicators present.

Since the sentence in your sample says “surface area”, it *should be clear that S.A. is an abbreviation and not S4A4 but if this is a test, out of of context, I suggest inserting an embedded transcriber’s note after “surface area” showing S.A. there as well (in UEB).

Thank you for this interesting question.

Lindy