Real wasabi is a type of radish.
Why it is true
Authentic wasabi is made from the root of Eutrema japonicum, a plant in the radish family. The spicy green paste served with sushi is often an imitation made from horseradish, mustard and green colouring.
Source: National Geographic
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.