D. Life is short; laugh and be merry.
The main idea of the poem is laugh and be merry. The poet repeats the word laugh three times throughout the poem. Repetition is often a good indicator of theme or the main idea. The poet does mention pride in being a human and fighting but these are not the main idea. Even though a song is mentioned, the song the poet is referring to is the song of laughter. Also, the title of the poem is laugh and be merry, usually titles are good indicators of theme or main idea.
