The vapour pressure of acetone at 20°C is 185 torr. When 1.2g of a non-volatile substance was dissolved in 100g of acetone at 20°C, its vapour pressure was 183 torr. The molar mass (g/mol) of the substance is:

The molar mass M₂ of non-volatile substance can be calculated from the following formula.

(p₀ - p) / p₀ = (W₂ × M₁) / (W₁ × M₂)

Here, p₀ = vapour pressure of acetone and p is the vapour pressure of the solution. M₁ and W₁ are the molar mass and mass of acetone. M₂ and W₂ are the molar mass and mass of a non-volatile substance.

Substitute values in the above expression.

(185 - 183) / 185 = (1.2 × 58) / (100 × M₂)

Hence, M₂ = 64.38 g/mol.

Thus, the molar mass of the non-volatile substance is 64 g/mol (approximately).

Hence, the correct option is B