Which of the following is not considered to be solid?

Solids are generally considered to be hard but a sponge is not as hard as other solids due to the presence of holes in its structure which traps air in it. But still, a sponge is a type of solid.

Sand is also solid as its every grain has a fixed shape and fixed volume.

Ice is a solid form of water.

Thus, they all are types of solids.

Hence, the correct option is (D).