Why An ideal op-amp is defined by a set of perfect properties: infinite open-loop gain, infinite input impedance, zero output impedance, and infinite bandwidth. Infinite bandwidth means the gain is flat at all frequencies, so it does not roll off at all. Real op-amps do roll off, typically at 6 dB per octave above a low corner frequency, characterized by a gain-bandwidth product, but that is a departure from the ideal model.
Watch out The choices describing a decrease with frequency describe a real device's open-loop response, where gain falls off above the corner frequency; the word 'ideal' in the question is the signal that no such limitation applies.
Ideal means infinite: infinite gain, infinite input Z, infinite bandwidth. Flat response, no rolloff.