20 g hemoglobin in a 1 L aqueous solution (A) at 300 K is separated...

The osmotic pressure $\pi$ of the solution is balanced by the hydrostatic pressure of the liquid column.

$\pi = \rho g h$

Given:
Density of solution, $\rho = 1000 \text{ kg m}^{-3}$
Acceleration due to gravity, $g = 10 \text{ m s}^{-2}$
Height difference, $h = 80.0 \text{ mm} = 0.08 \text{ m}$

$\pi = 1000 \times 10 \times 0.08 = 800 \text{ Pa} = 0.8 \text{ kPa}$

From the van 't Hoff equation for osmotic pressure:
$\pi = C R T = \dfrac{n}{V} R T$

Given:
Volume of solution, $V = 1 \text{ L} = 1 \text{ dm}^3$
Gas constant, $R = 8.3 \text{ kPa dm}^3 \text{ K}^{-1} \text{mol}^{-1}$
Temperature, $T = 300 \text{ K}$

Substituting the values:
$0.8 = \dfrac{n}{1} \times 8.3 \times 300$

$n = \dfrac{0.8}{2490} \text{ mol}$

The mass of hemoglobin is $w = 20 \text{ g} = 0.02 \text{ kg}$.
The molar mass $M$ in $\text{kg mol}^{-1}$ is:
$M = \dfrac{w}{n} = \dfrac{0.02}{\dfrac{0.8}{2490}}$

$M = \dfrac{0.02 \times 2490}{0.8} = \dfrac{49.8}{0.8} = 62.25 \text{ kg mol}^{-1}$

Rounding to the nearest integer, we get $62$.

Answer: $62$