Which of the following is NOT a suspending agent?

Calcium carbonate is the correct answer as it is not typically classified as a suspending agent. Suspended agents are substances used to maintain the stability of a mixture and prevent the solid particles from settling down too quickly. Methylcellulose, xanthan gum, and agar all serve this purpose effectively.

Methylcellulose is a popular suspending agent due to its ability to thicken solutions and improve the stability of suspensions. Xanthan gum is another effective suspending agent that provides viscosity and stabilizes emulsions and suspensions in various formulations. Agar serves as a gelling agent but also functions in a similar role as a suspending agent in certain contexts, particularly in microbiological media.

In contrast, calcium carbonate primarily acts as an antacid and a calcium supplement rather than a suspending agent. Its function does not align with the roles typically needed in formulations where maintaining dispersion of solids is critical. Therefore, the identification of calcium carbonate in this context as not being a suspending agent is accurate.