1. Use Fajans' rule
   The extent of covalent character in an ionic compound increases when:

   • the cation is small and/or highly charged  strong polarising power,
   • the anion is large and easily polarised.

   Since all compounds contain the same anion, $\text{Cl}^-$, we compare mainly the cations.

2. Compare the cations

   • $\text{K}^+$: alkali metal ion, large, low charge density $\Rightarrow$ weak polarisation $\Rightarrow$ compound is highly ionic.
   • $\text{Ba}^{2+}$: larger than many $2+$ ions, so though charge is $+2$, polarising power is moderate.
   • $\text{Co}^{2+}$: transition metal ion, smaller and with greater polarising power than $\text{Ba}^{2+}$, so $\text{CoCl}_2$ has more covalent character.
   • $\text{Ag}^+$: despite being only $+1$, it is a d-block cation with poor shielding by $d$-electrons, so it has appreciable polarising power and forms compounds with significant covalent character.

3. Expected ionic character order A rough order is: $\text{KCl} > \text{BaCl}_2 > \text{CoCl}_2 > \text{AgCl}$ in ionic character.

   Hence, the least ionic (i.e. most covalent among these) is: $\boxed{\text{AgCl}}$

4. Option check

   • A: $\text{CoCl}_2$  less ionic than KCl/BaCl$_2$, but not the least.
   • B: $\text{KCl}$  most ionic.
   • C: $\text{BaCl}_2$  quite ionic.
   • D: $\text{AgCl}$  least ionic.

Therefore, the correct option is D.