Which single feature most clearly distinguishes a Bohr orbit from a quantum-mechanical orbital?
a) A quantum orbital confines the electron to a fixed circular path, whereas a Bohr orbit does not.
b) A Bohr orbit has a definite radius, whereas a quantum orbital represents a probability distribution in space.
c) A Bohr orbit represents a three-dimensional probability cloud, whereas a quantum orbital is two-dimensional.
d) Bohr orbits can possess any energy value, while quantum orbitals have quantized energies only.