Hello!
The wave with the lowest frequency is the second one.
Why?
Frequency is defined as the number of times that a periodic phenomenon is repeated in a given time. Regarding waves, the frequency of a wave is the number of times that a wave repeats in a unit of time. Frequency is measured in Hertz (Hz=1/s).
The relationship between frequency and wavelength is inverse, meaning that longer wavelengths mean lower frequencies. From the given list, the wave with the longest wavelength is the second one, meaning that its frequency is the lower.
Have a nice day!