Answer:
Microwaves has the shortest frequency.
Step-by-step explanation:
In an electromagnetic spectrum, the different waves are arranged in an increasing or decreasing order of wavelength or frequency. Frequency of a wave is defined as the number of oscillations or vibrations produced. We have to write the name of wave having shortest frequency.
Frequency of infrared wave,

Frequency of microwaves wave,

Frequency of ultraviolet waves,

Frequency of x-rays,

So, from the above data it is clear that the microwaves has the shortest frequency.
Hence, the correct option is (b) " microwaves ".