Density of 3 M NaCl solution is $$1.25 g/mL$$. The molality of the solution is :

3 M NaCl solution with density 1.25 g/mL.

Determine the mass of solute in 1 L of solution

Molarity = 3 M means 3 moles of NaCl per litre of solution.

Molar mass of NaCl = 23 + 35.5 = 58.5 g/mol.

Mass of NaCl = 3 × 58.5 = 175.5 g.

Determine the mass of 1 L of solution

Mass of solution = Volume × Density = 1000 mL × 1.25 g/mL = 1250 g.

Calculate mass of solvent

Mass of solvent (water) = 1250 - 175.5 = 1074.5 g = 1.0745 kg.

Calculate molality

$$\text{Molality} = \frac{\text{moles of solute}}{\text{mass of solvent in kg}} = \frac{3}{1.0745} \approx 2.79 \text{ m}$$

The answer is Option 2: 2.79 m.