A is correct: 'u', at the beginning of a word is seen as consonant (when it comes to a/an). So, "a useful animal." vs. "an elephant."

'u' is tricky, because it can be pronounced a few different ways. In "useful", it is pronounced as if it started with 'y', a consonant: "yoos full". Therefore, you would say "a useful..." The word "unused" would take "an" as an article, because of its pronunciation - "uhn yooz d", which is pronounced like the short vowel. "An unused item". Another confusing word is "hour". You would say "an hour" even though the word starts with a consonant. This is because the 'h' is silent and it is pronounced just like "our", which starts with a vowel sound. You would say "a handle", though, because here the 'h' is pronounced.

I think the trick is to apply it from a speaking perspective rather than a written perspective, as fdmaxey explains well. The 'an' is used only to make the sound transition smoother in speech between the article and a following vowel sound. Sometimes you'd find strange 'an' uses in English from about a century ago, eg "an historical event" or "an hilarious joke".

Hi... Don't worry Allen you get use to it.

Yes [a] is correct