The sentence that makes correct use of an introductory comma is option A. Uh, how long should this process take?
There is a rule that states that words that aren’t part of the sentence must be separated by a comma. This words comment on the meaning of the sentence, instead. So, introductory words that are placed at the beginning of a sentence are set off from what follows by commas. Common introductory words include yes, no, well, oh, and okay.