Rhubarb leaves are toxic.
Why it is true
Rhubarb leaves contain high levels of oxalic acid, which can cause kidney failure and other serious problems if ingested in large quantities. Only the stalk is safe to eat, and even then it should be cooked.
Source: USDA
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.