1. Given data

• Molarity of solution $= 3\,\mathrm{M}$
• Density of solution $= 1.25\,\mathrm{g\,mL^{-1}}$
• Molar mass of $\mathrm{NaCl}$: $23 + 35.5 = 58.5\,\mathrm{g\,mol^{-1}}$

We need to find molality $m$.

2. Take 1 litre of solution

Since molarity is $3\,\mathrm{mol\,L^{-1}}$, in $1\,\mathrm{L}$ of solution:

$\text{moles of NaCl} = 3\,\mathrm{mol}$

3. Find mass of 1 litre of solution

Density $= 1.25\,\mathrm{g\,mL^{-1}}$

Volume $= 1\,\mathrm{L} = 1000\,\mathrm{mL}$

So, mass of solution is

$\text{Mass of solution} = 1.25 \times 1000 = 1250\,\mathrm{g}$

4. Find mass of solute ($\mathrm{NaCl}$)

$\text{Mass of NaCl} = 3 \times 58.5 = 175.5\,\mathrm{g}$

5. Find mass of solvent (water)

$\text{Mass of solvent} = \text{mass of solution} - \text{mass of solute}$

$= 1250 - 175.5 = 1074.5\,\mathrm{g}$

Convert into kg:

$1074.5\,\mathrm{g} = 1.0745\,\mathrm{kg}$

6. Calculate molality

Molality is moles of solute per kg of solvent:

$m = \frac{3}{1.0745}$

$m \approx 2.79\,\mathrm{m}$

7. Match with options

The correct option is:

$\boxed{\text{D: } 2.79\,\mathrm{m}}$