I'm almost completely sure the punctuation mark to that sentence is a period (.)
<u>Might be (!) though. So sorry if I am incorrect :(</u>
So the sentence would look like this with a period, "The boy ran as fast as he could." A period (.) is used at the end of any sentence that is intended to make a statement. As with other punctuation marks that end sentences, the period should be placed directly behind the last letter of the last word of the sentence.
