To find the mass percentage of the salt in the solution, we need to determine the mass of the salt and the mass of the solution.

Given that 20 g of common salt is dissolved in 180 g of water, the total mass of the solution is 20 g + 180 g = 200 g.

To find the mass percentage of the salt, we divide the mass of the salt by the mass of the solution and multiply by 100.

The mass of the salt is 20 g and the mass of the solution is 200 g.

(Mass of salt / Mass of solution) * 100 = (20 g / 200 g) * 100 = 0.1 * 100 = 10%

Therefore, the mass percentage of the salt in the solution is 10%.

Option-3 "10%" is the correct answer.