______ is the input of detector stage?

The correct option is 1

Concept:

1) IF signal - "IF" stands for Intermediate Frequency. In a superheterodyne radio receiver, the incoming RF signal is mixed with a local oscillator signal in the mixer and it is converted to an intermediate frequency which is easier to manipulate than the original RF frequency. This IF signal is then processed through various stages including the detector stage. So, IF signal is correct.

2) RF signal - An RF signal stands for "Radio Frequency" signal. There is a misunderstanding here, RF signal is not the direct input to the detector stage. Rather it's the input to the conversion stage happening before the IF amplifier and detector stage. So, the RF signal is incorrect.

3) Audio signal - An Audio signal is what is produced as output from the detector or demodulator stage. The detected audio signal is then further amplified by the audio amplifier stage. So, Audio signal is not correct.

4) Carrier Wave - A carrier wave is a type of signal that carries the information (modulated signal) from the transmission point to the receiving point. This carrier wave is modulated at the source with the target signal to create an RF signal. It's not the direct input to the detector stage. Hence Carrier wave is not correct.