Which of the following solutions have the same concentration?

Molarity (M) $=\frac{\text{ Weight of }\mathrm{NaOH}\times 1000}{\text{ Molecular weight of }\mathrm{NaOH}\times \mathrm{V}\left(\text{ in }\mathrm{mL}\right)}$

$=\frac{20\times 1000}{40\times 200}=2.5\mathrm{M}$

$\text{ (b) }\mathrm{M}=\frac{0.5\times 1000}{200}=2.5\mathrm{M}$

$\text{ (c) }M=\frac{40\times 1000}{40\times 100}=10\mathrm{M}$

$\text{ (d) }\mathrm{M}=\frac{20\times 1000}{56\times 200}=1.785\mathrm{M}$

Thus, 20 g NaOH in 200 mL of solution and 0.5 mole of KCI in 200 mL have the same concentration