A zero-level detector is a

ANS:B - comparator with a trip point referenced to zero

A trip point is the value of input voltage that would change the output voltage of a comparator from high to low or low to high. This means that for a zero level detector it needs an input of "0" volts hence "trip point is referenced at zero", so that it can produce change in the output which would indicated that a 0 level voltage is detected.