What is the physiological population density of Colorado?

The question makes no sense at all.

• there is no such thing as "arithmetic density";
• the normal definition of density is mass per unit of volume. In the context of an area on the surface of the earth, there is no volume - an area is 2-dimensional. There are no internationally agreed standards defining the depth of the state (unlike the height of air space above the surface).
• the question does not specify whether it refers to the state or river.