Answer:
The correct end punctuation is "Why do some trees drop their leaves in the autumn and others do not?".
Explanation:
"Evergreen trees retain their green color throughout the winter" is not an exclamation, merely a statement, so it should be finished with a period rather than an explanation point. "Look at that beautiful, red sugar maple tree" is an exclamation, not a question, so it should have an exclamation point rather than a question mark. "Plants stop making chlorophyll in the winter to save energy" should be finished with a period unless there's another part to the sentence. "Why do some leaves drop their trees in the autumn and others do not" is a question, so it is properly finished with a question mark.