Since I don't have a good computer to use for direct measurement, I'll use Wikipeda and compreviews.about.com as my sources for power consumption calculations.

First, in my area a "month" is considered to be a period of 30 days, so I'm going to use that for calculations. So 30*24=720 is the number of hours in a month. Next step is to determine power consumption of a device in watts and multiply.

So they say that on PC which is Energy Star certified (and I don't remember when I've seen one which isn't), power supply can't use more than 2 W when in stand-by and more than 4 W when in sleep. I'll take 4 W for calculation purposes.

So 4 W * 720 h = 2880 Wh = 2.88 kWh. We multiply that by the energy price, so we get:

2.88 kWh * 0.1135 $/kWh = 0.32688 $.

That means that all answers are A), since hibernate and power-off can't spend more than sleep.