Did I dissolve the sugar before adding the final three ingredients?
The question prompt asks about Selma trying to figure out what went wrong. This means we need an answer that would explain a mistake. The last option: Did I dissolve the sugar before adding the final three ingredients? does just that. It points out where a mistake could have occurred. If Selma didn't dissolve the sugar before adding the final ingredients, the lemonade would have a grainy consistency. The other options only answer what should could do differently next time rather than point to something that was done incorrectly this time.
