The English grammar is incorrect. The word "an" should appear as a choice since "an" is used before nouns beginning with a vowel.
The use of "an" in English is not dictated by the spelling, but rather by the pronunciation. The vowel "u" is sometimes pronounced as "oo" or "uh", both of which should be preceded by "an". Sometimes "u" is pronounced like the word "you", in which case it is actually starting with a consonant sound and would be preceded by "a". In the word "usual" the "u" is pronounced like "you" so "a" is correct. As a contrast, the word "unusual" starts with the "uh" sound and would take "an".
The indefinite article could present some difficulty for non-native speakers. (A) is used before words beginning with consonant sounds, and (an) is used in front of words starting with a vowel sound. The word, (usual), is pronounced with either a hu or you sound, so it takes (a) as its indefinite article. Compare this word with the sounds of umbrella or unusual, which both require the indefinite article, an.