The sentences with correctly used commas are:
-
Winston Churchill, Prime Minister of the United Kingdom, won the support of congressional leaders.
- The commas are correctly used to set off the appositive phrase "Prime Minister of the United Kingdom."
-
The man, who spoke to congressional leaders, was Winston Churchill.
- The commas are correctly used around the non-restrictive relative clause "who spoke to congressional leaders."
The other two sentences contain incorrect comma usage:
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During World War II, Winston Churchill, was Prime Minister of the United Kingdom.
- There should not be a comma after "Churchill."
-
Prime Minister Winston Churchill, won the support of congressional leaders.
- There should not be a comma after "Churchill."
So, the first two sentences are correct.