Linear spectrum (line spectrum) is a characteristic of which of the following?

Find the wavelength of radiation having frequency $4 \times 10^{11} \ kHz$.

Wavelengths of different radiations are given below:
$\lambda(A) = 300 \ nm$
$\lambda(B) = 300 \ \mu m$
$\lambda(C) = 3 \ nm$
$\lambda(D) = 30 \ \mathring{A}$
Arrange these radiations in the increasing order of their energies.

The velocity of $\alpha-$rays is approximately

Electromagnetic radiations of frequency $\nu$ consist of a stream of particles called photons. Which of the following statements is / are true about photons:

$X$-rays are produced due to