Answer: B. Life is short; laugh and be merry.
Step-by-step explanation: In the given poem "Laugh and be Merry" by John Masefield, we can see that the main idea is the one given by option B: Life is short; laugh and be merry. One way to identify the main idea in the given lines is to pay attention to the repetition (literary device that consists in repeat words or phrases to emphasize them), in this case we cans see the word "laugh" repeated three times, so it helps to convey the main idea.