The answer to this question is C
An isotope of an element is another element that has the same number of protons but a different number of neutrons (for instance

and

)
You have 45 kJ of heat added and for every 40.6 kJ added, one mole is able to be vaporized. Find how many moles you can vaporize.
45 kJ / 40.6 kJ = 1.1 moles
Water is roughly 18 g per mole so 1.1 moles x 18 g per mol = 19.95 grams vaporized
Hope I helped!