Final answer:
The spell to reduce solid objects into tiny pieces is known as Reducto, or the Reductor Curse, in Harry Potter lore.
Step-by-step explanation:
The incantation for the spell that reduces solid objects into tiny little pieces is Reducto. This spell is known as the Reductor Curse in the wizarding world of Harry Potter. When cast, it can disintegrate objects or blast holes in them. The other options listed, such as Expelliarmus, Sectumsempra, and Diffindo, serve different purposes. Expelliarmus is used to disarm an opponent, Sectumsempra is a spell that causes cuts and bleeding, and Diffindo is the Severing Charm which is used to slice objects.