In “The Silly Song,” Dopey plays a drum.
The sequence, also called “The Dwarfs' Yodel Song,” shows the seven dwarfs entertaining Snow White after she joins them at their cottage. Dopey's comic musical moment is a drum solo, performed amid the lively dancing and slapstick action. He becomes distracted while chasing a housefly, turning the performance into one of the scene's visual jokes.
The other instruments in the number can make the question seem trickier. Grumpy gets an organ solo, while Doc plays a lute. Dopey's instrument is therefore not the banjo, flute, or triangle listed among the alternatives. The drum is part of his character's energetic, mischievous comedy rather than a formal musical performance.
“The Silly Song” was written for Walt Disney's 1937 animated feature and is one of the film's ensemble numbers. It is followed by Snow White's “Someday My Prince Will Come,” linking the dwarfs' comic entertainment to the film's romantic storyline.