Question

what material makes up the skeleton of hexactinellid sponges?
silica
calcium carbonate
sand

Explanation:

Hexactinellid sponges, also known as glass sponges, have a skeleton made of silica - based spicules. Silica is a common material in their skeletal structure. Calcium carbonate is more typical of other types of sponges (like calcareous sponges) or other marine organisms (such as corals). Sand is not a biological material that forms the skeleton of these sponges.

Answer:

silica