A subordinating conjunction introduces a subordinate/dependent clause. Here, the subordinating conjunction is after.
I believe the answer is: Charlotte's swept the floor after she finished cooking
Subordinating conjunction refers to a combination between two phrases in which one of the phrases acted as an independent phrase while the others acted as a dependent phrase.This conjunction usually created to show a transition between two ideas or situation.
Ok, first let's narrow down what a subordinating conjunction even is..
SUBORDINATION CONJUNCTION = after, before, although, etc.
So, with that said, which one do YOU think it is?
The answer to your question is B. Charlotte swept the floor AFTER she finished cooking.
It contains the subordinate conjunction "after".
Hope this helps 🙂