I have no problem with your answer #3, and it could be argued that the question is not sufficiently precise. It depends what you consider the "inputs" to be. At the motor control level, the hex rotor has 6 motors so there are 6 motor speeds that can be independently set. However hexrotors often use the same kind of control handset as a quad rotor, with sticks that control motion:up/down, roll, pitch, yaw. Considered at this level there are 4 "inputs" which are mapped to the 6 motor speeds. This introduces constraints and the 6 motor speeds are no longer independent.
The vehicle itself is a rigid-body moving in 3D space so it has 6 degrees of freedom. Assuming that all propellor axes are parallel, only 4 of those degrees of freedom are directly accessible, just like for a quad rotor. Translational motion is only accessible as a consequence of roll/pitch motion.