Which of the following electromagnetic waves have the minimum frequency?

Given below are some famous numbers associated with electromagnetic radiations in different contexts in physics. State the part of the electromagnetic spectrum to which each belongs.
$(a)$ $21 \; cm$ (wavelength emitted by atomic hydrogen in interstellar space).
$(b)$ $1057 \; MHz$ (frequency of radiation arising from two close energy levels in hydrogen; known as Lamb shift).
$(c)$ $2.7 \; K$ (temperature associated with the isotropic radiation filling all space; thought to be a relic of the 'big-bang' origin of the universe).
$(d)$ $5890 \; \mathring{A} - 5896 \; \mathring{A}$ (double lines of sodium).
$(e)$ $14.4 \; keV$ (energy of a particular transition in $^{57}Fe$ nucleus associated with a famous high-resolution spectroscopic method, Mossbauer spectroscopy).

If a source of electromagnetic radiation having power $15 \text{ kW}$ produces $10^{16}$ photons per second,the radiation belongs to which part of the spectrum? (Take Planck constant $h = 6 \times 10^{-34} \text{ Js}$)

The frequencies of $X$-rays,$\gamma$-rays,and ultraviolet rays are $a, b$,and $c$ respectively. Which of the following relations is correct?

Write the range of frequencies of visible light.

Pick out the longest wavelength from the following types of radiations.