The formula to calculate the moles of solute needed to prepare a solution of a given concentration is:

Moles of solute = Concentration (in moles/liter) x Volume of solution (in liters)

In this case, we have:

Concentration = 0.2 M (given)

Volume of solution = 300 mL = 0.3 L (converting milliliters to liters)

Substituting the values in the formula, we get:

Moles of NaOH = 0.2 M x 0.3 L = 0.06 moles

Therefore, 0.06 moles of NaOH are needed to prepare 300 mL of a 0.2 M NaOH solution.