Among the following pairs of solutions, isotonic solutions are

Isotonic solutions mean two or more solutions that have same number of species or ions with same concentration.

$\mathrm{For} \mathrm{Ca}{\left({\mathrm{NO}}_3\right)}_2,\mathrm{i}=1+2=3(1{\mathrm{Ca}}^{+},2{\mathrm{NO}}_3^{-}\mathrm{ions})$

$\mathrm{For} {\mathrm{Na}}_2{\mathrm{SO}}_4,\mathrm{i}=2+1=3(2{\mathrm{Na}}^{+} \mathrm{and} 1 {\mathrm{SO}}_4^{-2})$

The species $\mathrm{Ca}{\left({\mathrm{NO}}_3\right)}_2 \& {\mathrm{Na}}_2{\mathrm{SO}}_4$ have same Vant Hoff factor (i) and same concentration (0.1M).

Hence, the pair $0.1\mathrm{M} \mathrm{Ca}{\left({\mathrm{NO}}_3\right)}_2 \mathrm{and} 0.1 \mathrm{M} {\mathrm{Na}}_2{\mathrm{SO}}_4\mathrm{to} \mathrm{be} \mathrm{isotonic} \mathrm{solutions}.$