Waves will have the highest speed in ...

alr at o'c

alr at 5°C

air at 20°C

alr at 30°C

Waves will have the highest speed in air at 30°C.

Explanation:

The speed of wave depends on the temperature. At higher temperature, the molecules starts vibrating with more velocity. As a result, the speed of the wave increases.

It also depends on the frequency and the wavelength of the wave. If either frequency or the wavelength increase, the speed of the wave will also increase.

So, at 30 °C the wave will have highest speed.

Hence, the correct option is (d).