The correct option is D: two quantum mechanical spin states which have no classical analogue. The concept of electron spin with quantum numbers +1/2 and -1/2 indeed represents two intrinsic angular momentum states of an electron, but it doesn't directly correspond to the physical picture of the electron spinning around its own axis like a planet, either clockwise or anticlockwise. This description of spin is a quantum mechanical property that has no classical analogue, meaning that it cannot be accurately described using classical physics concepts. These two states can be associated with the magnetic moment of the electron pointing in two opposite directions, often referred to simplistically as "up" and "down" in the context of its orientation in an external magnetic field, but it's important to understand this is a representation that helps visualize their quantum mechanical behavior regarding their interaction with magnetic fields, rather than a literal physical spinning.