Answer: A) Money is taken out of your paycheck before you pay taxes on it.
I'm assuming your teacher is talking about tax deferred retirement plans. This is where the money you put into the retirement account isn't part of your taxable income (but the money coming out is taxed). In short, the tax is not applied to the input but it is applied to the output.
This is in contrast to a tax exempt retirement account where the input money is taxed but the output money is not taxed. This is of course a very simplified picture. The actual tax rules and various investment options are much more complicated.