Molarity or molar concentration is a measure of the concentration of a chemical species, of a solute in a solution, using as units, number of moles, and volume in liters. The formula for Molarity is:
M = n/V
Where:
n = number of moles, 3 moles
V = volume in Liters, 6 Liters
Now adding these values into the formula:
M = 3/6
M = 0.500 M is the molarity or molar concentration, 2nd option