Isoelectronic species are atoms or ions that contain the same number of electrons (NCERT Section 3.7.1). We calculate the total number of electrons for the species in the correct option:

1. Ca²⁺: Atomic number (Z) of Calcium is 20. It loses 2 electrons. Total electrons = 20 - 2 = 18.
2. Ar: Argon is a noble gas with atomic number 18. Total electrons = 18.
3. K⁺: Atomic number of Potassium is 19. It loses 1 electron. Total electrons = 19 - 1 = 18.
4. Cl⁻: Atomic number of Chlorine is 17. It gains 1 electron. Total electrons = 17 + 1 = 18.

Since all species possess 18 electrons, they are isoelectronic. (Note: In Option 3, Na⁺, Mg²⁺, and F⁻ have 10 electrons, but O⁻ has 9 electrons; O²⁻ would have been isoelectronic).