The answer is the option A.molarity.
M represents molarity, and the question requires a direct application of the definition of molarity.
Molarity is the concentration measurement defined as number of moles of solute per liters of solution, this is M = number of moles of solute / volume of the solution in liters.
Which is the expressed by [M] = mole / liter
The symbol [ ] is used to mean units, son [M] means units of M. So if the units of M is mole / liter, this is molarity.