A.) "I know what happened." whispered the boy, "I saw the whole thing from my window."
B.) "I know what happened" whispered the boy. "I saw the whole thing from my window."
C.) "I know what happened," whispered the boy. "I saw the whole thing from my window."
D.) "I know what happened." whispered the boy. "I saw the whole thing from my window."
The answer is C. The sentence needs to have a comma because that isn't the end of the sentence. It continues over to the "I saw the whole thing from my window." Which would be the end.