An oscillator is nothing but an amplifier with:
A) Positive feedback
B) Negative feedback
C) Large gain
D) Low gain

Hint: An oscillator is an electronic device which generates the alternating voltage. In this type of device the output power is given back to the input. This is called positive feedback. An oscillator works on the principle of positive feedback. Oscillator gives the desired output voltage with the help of positive feedback.

Complete step by step answer:
An oscillator is a device which gives alternating voltage with the help of positive feedback. In an oscillator using feedback it is important to keep the output voltage stable. So the closed loop must have a gain of unity i.e. 1. This closed loop acts as a positive feedback which collects the voltage from output and gives back to its input.
The gain provided by the amplifier should exactly match with the closed loop which is the positive feedback. The other type of feedback is called the negative feedback. In the negative feedback the amplifier subtracts some fraction of output from its input. The summing circuit acts as a subtractor. Whereas the positive feedback adds a part of fraction of output to input. The output gain is controllable using the amplifier. Also the gain of the positive feedback should be unity for a closed loop so as to keep the output voltage stable.

Note: An oscillator does give the gain but the value of gain is not differentiable. Here its principle of working is important on which an oscillator works. The positive feedback used in an oscillator is because of the stable output voltage by keeping the gain of the closed loop equals to unity.