We have been given the speed and wavelength of the microwave in air. When the microwave enter a plastic food container, the speed reduces.
By using the formula, c = ƒ x λ, we know speed is directly proportional to wavelength. If the speed decreases, the wavelength will also decrease. The only options which have a lower wavelength than 12 cm are A, B and C. Therefore the wavelength would be 8cm.
By using the formula, ƒ = c/λ, we can calculate the new frequency. We need to convert 8 cm to metres. 8 cm = 0.08 m
ƒ = c/λ
ƒ = 2.0 x 108 / 0.08 = 2.5 x 109