Given density of the solution is 1.44 g/cm³.

Given density of the solution is $1.44 \, \text{g/cm}^3$.
Since $1 \, \text{L} = 1000 \, \text{cm}^3$:

$$\text{Mass of 1 L of solution} = \text{Density} \times \text{Volume}$$ $$\text{Mass of 1 L of solution} = 1.44 \, \text{g/cm}^3 \times 1000 \, \text{cm}^3 = 1440 \, \text{g}$$

The percentage purity is $65\%$.

$$\text{Mass of pure HNO}_3 = \text{Mass of solution} \times \text{Purity percentage}$$ $$\text{Mass of pure HNO}_3 = 1440 \, \text{g} \times 0.65 = 936 \, \text{g}$$

The formula mass of HNO$_3$ is $63 \, \text{g/mole}$.

$$\text{Moles of HNO}_3 = \frac{\text{Mass of pure HNO}_3}{\text{Formula mass}}$$ $$\text{Moles of HNO}_3 = \frac{936 \, \text{g}}{63 \, \text{g/mole}} \approx 14.857 \, \text{moles}$$

Concentration (Molarity) is the number of moles per litre of solution.

$$\text{Concentration} = \frac{\text{Moles of HNO}_3}{\text{Volume of solution (L)}}$$ $$\text{Concentration} = \frac{14.857 \, \text{moles}}{1 \, \text{L}} \approx 14.86 \, \text{mol/L}$$

The concentration of the stock solution is approximately $\text{14.86 mol/L}$.