A single strand of spaghetti is a 'spaghetto'.
Why it is true
In Italian, the word 'spaghetti' is plural. Therefore, one single piece is referred to as a 'spaghetto'.
Source: Linguistics
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.