The emission spectrum of a gaseous atom is line-based and not continuous. Why?

What is the difference in the line spectrum of different atoms compared to the hydrogen atom?

The energy of the third orbit of $Li^{2+}$ ion (in $J$) is

When the electron passes from the energy state nearest to the nucleus to the third energy level,it will:

How many emission spectral lines are possible when a hydrogen atom is excited to the $n$th energy level?

The following figure shows the spectrum of an ideal black body at four different temperatures. The number of correct statement$(s)$ from the following is $..............$.
$A$. $T_4 > T_3 > T_2 > T_1$
$B$. The black body consists of particles performing simple harmonic motion.
$C$. The peak of the spectrum shifts to shorter wavelength as temperature increases.
$D$. $\frac{T_1}{v_1} = \frac{T_2}{v_2} = \frac{T_3}{v_3} \neq \text{constant}$
$E$. The given spectrum could be explained using quantization of energy.