In the exercise it says i have a typo and the correct is AN historic ! )))))))))
Some dialects of English put "an" before "historic". I'm from Chicago and I do (though I'll admit it's a little rare here).
Aside from the obvious ones like hour, herb, etc., this is the only one I can think of. E.g., I'd never say "This book is an history of Egypt." But "historic" gets the "an" and the "h" becomes silent.
Maybe it has something to do with the accent being on the second syllable of "historic" but not of "history"?