The osmotic pressure of a living cell is 12 atm at 300 K. The...

For osmotic pressure, using the van 't Hoff equation: $\pi = iMRT$.
The living cell has osmotic pressure of 12 atm.
For an isotonic NaCl solution, the osmotic pressures must be equal.
For NaCl solution: $\pi = i \times M \times R \times T$, where $i = 2$ (complete dissociation into Na⁺ and Cl⁻).
$12 = 2 \times M \times 0.08 \times 300$
$12 = 48M$
$M = 0.25$ mol/L
Molar mass of NaCl = 23 + 35.5 = 58.5 g/mol.
Concentration in g/L = $0.25 \times 58.5 = 14.625 \approx 15$ g/L