Sentence Correction (SC) | Re: An abscess swelling should go down

Definitely we cannot use 'and' here so D and E are gone

In C - 'did' is incorrect
In B - tense 'having been' is correct
A is correct

Specific to your doubt

I think 'but' should have a comma when you have two independent clauses
i.e two stmnts independently make sense

Here in this sentence 'comma but' is not an issue so we cannot eliminate B and C because of comma

Check below example for usage of comma but

(without comma)
Grammar is boring but necessary

(with comma)
Grammar is
...

---