Which sentence is punctuated correctly? A. When the milk carton fell, it spilled but I picked it: up before all the milk flowed onto the floor. B. When the milk carton fell, it spilled; but I picked it up before all the milk flowed onto the floor. C. When the milk carton fell, it spilled, but, I picked it up before all the milk flowed onto the floor. D. When the milk carton fell, it spilled but I picked it up. before all the milk flowed onto the floor.

The sentence that is puctuated correctly is B. When the milk carton fell, it spilled; but I picked it up before all the milk flowed onto the floor.

Explanation:

Sentence B is punctuated correctly because the following puctuation rules are respected:

there are two related independent clauses joint by a semicolon -when the milk carton fell, it spilled and but I picked it up before all the mil flowed onto the floor.
the adverbial clause -when the milk carton fell- is separated from the rest of the sentence with a comma.