The odd one out is C, litchi. The other four fruits (pear, apple, guava, and orange) are all pomes, which are fruits that have a core with seeds inside. Litchi is a drupe, which is a fruit that has a single seed surrounded by a fleshy outer layer.
Pome fruits are typically round or oval in shape, with a thick skin that is often red, green, or yellow. The flesh of a pome fruit is usually white or cream-colored, and it is often juicy and sweet. The core of a pome fruit is made up of several seeds, which are usually hard and inedible.
Drupe fruits are typically round or oval in shape, with a thin skin that is often red, purple, or black. The flesh of a drupe fruit is usually soft and juicy, and it surrounds a single seed. The seed of a drupe fruit is usually hard and inedible.