http://www.freerice.com/index.php
These sorts of things annoy me a little. I mean, it's a good thing that they are donating food, but why wait for people to answer a quiz? Why not just give hungry people as much food as you can?
Because of that dilemma, half the time these sorts of things are a hoax (this one looks legit, but I'm thinking of stuff like email forwards that claim the cancer society will donate more money based on how many people get the email.) The other half of the time, well you have to bear in mind that the sponsors paying for the donations almost always have a donation limit that isn't mentioned to people taking the quiz (or shopping there if it's a donation per shopper, or whatever.) So after the donation limit has been reached (and it's usually reached pretty quickly) then everyone else taking the quiz is just wasting their time and erroneously thinking they are helping fight hunger.