Which of the following is used to reduce or eliminate intermodulation interference in a repeater caused by a nearby transmitter?
Why Intermod in a repeater often forms inside its own transmitter: RF from a nearby transmitter travels backward down the feed line into the final amplifier, where the nonlinear output stage mixes it with the repeater's carrier and radiates the sum and difference products. A circulator (isolator) is a ferrite device that passes power forward to the antenna but routes any reverse-traveling energy into a dummy load, so the intruding signal never reaches the PA. The termination must be a good, properly rated load, since that is where the unwanted power is dissipated.
Watch out A band-pass filter 'in the feed line between the transmitter and receiver' describes no real path; cavity filters do help duplexers, but they are placed in each antenna line, not between transmitter and receiver. Class C stages are strongly nonlinear and would generate more mixing products, not fewer.
Circulator = one-way street with a trash can: forward power out, reflected and intruding RF into the load.