Question

An oscillator uses (a) positive feedback. (b) negative feedback. (c) both positive and negative feedback. (d) no feedback.

Answer 1

Actually there no such rule that you have to give negative feedback to make a stable oscillator. There is a rule given by Barkhausen that the phase difference between input and feedback of amplifier should be integral multiple of 2π.  So you can make an oscillator by giving input to inverting terminal also, but the feedback circuit should also provide phase difference of π so as to fulfill the Barkhausen’s criteria.