The question says "All the students told the truth" meaning that at least one of the statements each student makes is true.

Now look at what darcy says: "It was charlotte" "It was bella"

only one of the statements are true here. It cant be both because only one of the students put the spider on the chair. It can't be neither because the question says that one of the statements has to be true.

This means that it has to be either Charlotte or Bella So we've narrowed it down to either Charlotte or bella

Now look at what archie says: "It wasn't edward" "It was bella"

If the statement "it was bella" is true that means the other statement "it wasn't Edward" is false.

That's not possible because

"It wasn't edward" (false) means that it was edward who put the spider on the chair "It was bella" (true) means that it was bella who put the spider on the chair

It can't be both students so the statement "It was bella" is not true

That means that the statement "It wasn't edward" is true

"It wasn't edward" (true) means it wasn't edward "It was bella" (false) means it wasn't bella

We've already narrowed it down to either Charlotte or bella from darcys statement so if it wasn't bella that means it was Charlotte

Sorry for the long post, I know its quite a mouthful but if you read it a few times I hope it will make sense

Also what subject is this because I've never seen these types of questions before