Molarity or M is a unit of concentration and is the ratio of number of moles to the volume of the solution (in liters). 

That is, molarity = number of moles / volume of solution

Here, the volume of solution is given as 250 ml. There are 1000 ml in a liter of a solution. Therefore the volume of solution is 250 ml/1000 L = 0.25 L.

The molarity of the solution is 0.15 M.

Thus, 0.15 = number of moles / 0.25 L

or, number of moles = 0.15 mol/L x 0.25 L = 0.0375 moles.

Thus, there are 0.0375 moles of potassium hydroxide in the solution when we have 250 mL of 0.15 M solution.

Since the molar mass of potassium hydroxide is 56.1 g, we have 2.1 g (0.0375 moles x 56.1 g/mole) of potassium hydroxide.

Hope this helps.