What Is the Crucial Characteristic of an Einstein Solid That $\Omega(U, N)$

What is the crucial characteristic of an Einstein solid that makes it easier to analyze in the context of this chapter than most other kinds of thermodynamic systems?

A) The atoms are arranged in a regular, cubic lattice.
B) The atoms are identical.
C) Its microstates are comparatively easy to count.
D) Each atom's energy levels are equally spaced.
E) $\Omega(U, N)$ is always reasonably small.
F) Other (specify) .