Quantum mechanics<span> (QM -- also known as </span>quantum physics<span>, or </span>quantum<span> theory) is a branch of </span>physics<span> which deals with physical phenomena at nanoscopic scales where the action is on the order of the Planck constant. It departs from classical </span>mechanics<span> primarily at the </span>quantum<span>realm of atomic and subatomic length scales.</span>
"poetic description" is the only real option from the list that is not common in expository writing, although careful descriptions can indeed be used as long as the point remains clear and forceful.
