B.

Cl < P < Mg < Ca

With the increase in the number of electron in the same shell, the atomic radii decrease due to increase in effective nuclear charge. However, atomic radii increases, as the number of shells increases. Thus, on moving down a group atomic radii increases.
The electronic configuration of the given elements is
Mg₁₂ = [Ne] 3s²
Ca₂₀ = [Ar] 4s²
P₁₅ = [Ne]3s² 3p³
Cl₁₇ = [Ne] 3s² 3p⁵
In Mg, P and Cl, the number of electrons is increasing in the same shell thus, the order of their atomic radii is
Cl <P < Mg
In Ca, the electron is entering in the higher shell, thus, it has the highest atomic radii among the given. Thus, the order of radii is
Cl < P < Mg < Ca

B.

CsF

The order of size of given cations is 
Li⁺ < Na⁺ < Cs⁺
and the order of the size of given anions is 

A.

[Ne] 3s² 3p³

Ionisation energy usually increases from left to right in a period with a decrease in atomic size and decrease from up to down in a group with an increase in atomic size.
[Ne] 3s² 3p³     = Group V
[Ne] 3s² 3p²     = Group IV

[Ar] 3d¹⁰, 4s² 4p³ = Group V
[Ne] 3s² 3p1 = Group III

B.

Li⁺

Isoelectronic species contain the same number of electrons Be²⁺ contains 2 electrons. Among the given options, only Li⁺ contains 2 electrons and therefore, it is isoelectronic with Be²⁺.
H⁺ --> no electron; Na⁺ --> 10e^-
Li⁺ --> 2e^-;
Mg²⁺ --> 10 e^-
Hence, Be²⁺ is isoelectronic with Li⁺

C.

ii, iv and v

The value of l varies from 0 to n-i and the value of m varies from -l to +l through zero.
the value of 's'  Which signifies the spin of the electron. The correct sets of quantum number are following: