AP EAMCET Engineering 2013 Solved Paper

Two solutions are isotonic if they have same molar concentrations of the particles.
(a) 0.15 M NaCl and 0.1 M Na2SO4
NaCl is an electrolyte which dissociates to give 2 ions, thus concentration of ions in the solution 0.30 M.
Similarly for Na2SO4 (3 ions), the concentration of ions in the solution = 0.30 M. Hence, both are isotonic.