My flight instructor asked me if throttle can be considered as a primary flight control?
14 CFR 23.673 - Primary flight controls defines primary flight controls as:
Primary flight controls are those used by the pilot for the immediate control of pitch, roll, and yaw.
While a throttle could be used to effect changes in aircraft attitude (depending on configuration and other factors), the effect could hardly be called immediate/direct. So, throttle could hardly be called a primary flight control; and as @Ron pointed out, there are gliders.
Note: I'm aware that aircraft have been controlled solely using throttle. But it is an exception rather than the norm.
Yes, according to wikipedia, the throttle is considered to be a primary flight control, along with
a control yoke (also known as a control column), centre stick or side-stick (the latter two also colloquially known as a control or joystick), governs the aircraft's roll and pitch by moving the ailerons (or activating wing warping on some very early aircraft designs) when turned or deflected left and right, and moves the elevators when moved backwards or forwards rudder pedals, or the earlier, pre-1919 "rudder bar", to control yaw, which move the rudder; left foot forward will move the rudder left for instance. throttle controls to control engine speed or thrust for powered aircraft.
It is considered to be a primary flight control because it makes a big difference to the aircraft, a plane with no throttle can't fly!