From the Super Mario Wiki
When the player decides to stop here, he or she meets Bob-omb, a member of an underground organization. Its quest, Find the Password, involves telling it the password of the Bob-ombs. Since the player does not know it, Bob-omb tells him or her to go to the Item Shop and buy a Bob-omb figure. Once the player is at the Item shop, and buys the figure from Paratroopa, the latter reveals the password: "Baboom." If the player returns to Bob-omb Avenue and tells Bob-omb the password, it rewards him or her with the Shroom Slide Gaddget.