Why do solids maintain their shape, whereas fluids do not?
a. because the molecules in solids maintain a regular pattern and move very rapidly.
b. because solids are denser than fluids.
c. because solids are under greater pressure than fluids.
d. because the molecules in solids maintain a regular pattern and only vibrate, or move very slowly.

Option (d) is the correct answer.

Explanation:

In solids, the molecules are held more tightly due to the existence of strong intermolecular forces of attraction between its particles.

Hence, they have a definite shape and volume. So, these molecules are arranged in a regular pattern and they are able to vibrate at their position.  

On the other hand in liquids, molecules are slightly away from each other as they are held by less strong intermolecular forces of attraction. Hence, liquids do not have a definite shape but they tend to occupy the shape of container in which they are placed.

Thus, we can conclude that solids maintain their shape, whereas fluids do not because the molecules in solids maintain a regular pattern and only vibrate, or move very slowly.