The size of isoelectronic species — F^–, Ne and Na⁺ is affected by

(a) nuclear charge (Z)

(b) valence principal quantum number (n)

(c) electron-electron interaction in the outer orbitals

(d) none of the factors because their size is the same.

The size of an isoelectronic species increases with a decrease in the nuclear charge (Z).

For example, the order of the increasing nuclear charge of F^-, Ne, and Na⁺ is as follows:

F^-(Z=9) < Ne(Z=10) < Na⁺(Z=11)

Therefore, the order of the increasing size of F^-, Ne and Na⁺ is as follows:

Na⁺ < Ne < F^-

Hence, the correct option is - (a) nuclear charge (Z).