There are a number of ways to express concentration of a solution. This includes molarity. Molarity is expressed as the number of moles of solute per volume of the solution. The concentration of the solution is calculated as follows:
Molarity = 2.0 mole / L solution
2.0 mole / L solution ( 0.50 Liters ) = 1 mole solute
The correct answer is the third option. One mole of solute needed to make 0.50 liters of 2M solution.