The correct answer is D: 6 dB. When the operating frequency of an ideal parabolic reflector antenna is doubled, the gain increases by 6 dB. This is because gain is proportional to (diameter/λ)², so doubling frequency (halving wavelength) quadruples the gain ratio, which is 6 dB. For parabolic reflectors, gain = (πD/λ)² × efficiency, where D is diameter and λ is wavelength. If frequency doubles, wavelength halves. So gain becomes (πD/(λ/2))² = (2πD/λ)² = 4 × (πD/λ)². This is 4× the original gain, which is 6 dB (10 log₁₀(4) = 6 dB). This relationship is important for understanding how parabolic antennas scale with frequency. Higher frequencies allow smaller dishes to achieve the same gain, or the same dish to achieve higher gain.