Which of the following order is correct for covalent character (if the cation sizes are identical in pairs)?

left to right in period covalent character increasing and top to down in group ionic character decreasing.
Hence the result,
$\begin{array}{l}{\mathrm{CaCl}}_2<{\mathrm{PdCl}}_2\\ \mathrm{NaCl}<\mathrm{CuCl}\\ \mathrm{RbCl}<\mathrm{AuCl}\end{array}$
Hence, correct option is D.