Q.Assertion: The spectrum of He$^+$ is expected to be similar to that of hydrogen. Reason: He$^+$ is also one electron system.

Splitting of spectral lines in an electric field is the Stark effect; splitting in a magnetic field (a different phenomenon) is the Zeeman effect.

When atoms that would normally emit a single spectral line are placed in an external field, that line can split into several closely spaced lines because the field removes some of the degeneracy of the atomic energy levels:

• In a magnetic field, this splitting is called the Zeeman effect.
• In an electric field, this splitting is called the Stark effect.
• The Compton effect refers to a completely different phenomenon — the increase in wavelength of X-rays scattered by electrons. …

Splitting of spectral lines under an applied electric field is called the Stark effect.

When atoms that are emitting or absorbing radiation are placed in a strong external electric field, their spectral lines split into several closely spaced components. This is the Stark effect, discovered by Johannes Stark, and it arises because the electric field perturbs (shifts) the energy levels of the atom depending on their orientation relative to the field.

…

Statements (I) and (II) are factually incorrect; only statement (III) is correct.

Examine each statement:

(I) 'The number of electrons in an atom is equal to the number of neutrons in it' - this is FALSE. In a neutral atom, the number of electrons equals the number of PROTONS (the atomic number Z), not the number of neutrons. Electrons equal neutrons only coincidentally for a few specific atoms, not as a general rule.

(II) 'The number of protons in an atom is called its mass number' - this is FALSE. The number of protons is called the atomic number (Z). The mass number (A) is the sum of the number of protons AND neutrons (A = Z + N). …