Which of the following electronic transitions in a hydrogen atom will require the highest energy?

If an electron moves from an excited state to the ground state in multiple steps in the hydrogen spectrum,and the number of lines in the visible region is $4$,then the number of lines in the infrared region is:

The frequency of radiation emitted when the electron falls from $n = 4$ to $n = 1$ in a hydrogen atom will be (Given $h = 6.625 \times 10^{-34} \, J s$):-

The work function for a metal is $4 \, eV$. To emit a photo electron of zero velocity from the surface of the metal,the wavelength of incident light should be : ................ $\mathring{A}$

The ratio of the difference in energy between the first and second Bohr orbits to that between the second and third orbits is

$A$ $50 \, W$ bulb emits monochromatic red light of wavelength $795 \, nm$. The number of photons emitted per second by the bulb is $x \times 10^{20}$. The value of $x$ is $......$.
$[ \text{Given} : h=6.63 \times 10^{-34} \, J \cdot s \text{ and } c=3.0 \times 10^{8} \, m \cdot s^{-1} ]$