Answer:
2- When the Revolutionary War ended,
Explanation:
The correct sentence should read as follows: When the Revolutionary War ended, George washington could have become king of America.
As the adverbial of time is in initial position (which is a marked or non-typical position), you must place a comma after it. The normal word order of a sentence should be:
George washington could have become king of America when the Revolutionary War ended.
(Subject + Verb + Complement)
However, for some reason (which could be for the sake of emphasis), the adverbial of time or Complement "when the Revolutionary War ended" has been moved to initial position, and this movement must be singaled with a comma.
To sum up, you always use a comma after an adverbial which is in initial position.
As to the other options, they all must be discarded because the punctuation is not appropriate:
1- <em>When the Revolutionary War ended</em>: there is no punctuation, as the comma after this clause is missing.
3- <em>When the Revolutionary War, ended</em>: you cannot separate a subject and a verb with a comma. It is not grammatically correct.
4- <em>When the Revolutionary War, ended,</em>: Same case as 3, although the comma after "ended" is correct, the one before "ended" is not. You cannot separate a subject and a verb with a comma.
