Select the most appropriate option to fill in the blank. The student gave his…

The choice between "a" and "an" depends on the SOUND that starts the next word, not the letter it is spelled with. A word beginning with a consonant sound - including a "y"-sound spelled with a vowel letter - takes "a"; a word beginning with a vowel sound - including a silent "h" - takes "an".

The word "useful" is pronounced starting with the consonant sound "y" (as in "you"), even though it is spelled with the vowel letter "u". Since the next word begins with a consonant sound, the article before it must be "a", giving "a useful feature".

• "The" would require the feature to already be specific or uniquely identified - one mentioned earlier or the only one that exists. Here the feature is being introduced for the first time with no such identification, so "the" does not fit.

• "An" applies only when the following word begins with a vowel sound, such as in "an hour" (silent "h") or "an honest man". Since "useful" begins with the consonant sound "y", "an" is ruled out here.

• Leaving the article out entirely ("no article") would leave the singular countable noun "feature" without any determiner, which is not grammatical in this position.

Hence the blank must be filled with "a", giving "...using a useful feature of his laptop."