WORDS that define a noun as specific or unspecific.The definite article 'the' is USED with the name of things that are unique or already mentioned before.The article 'a' is used with the name of things that are not specific.The article 'an' is used with the words having the first letter as a vowel and having the sound of a vowel.If a WORD starts with a vowel but has the sound of a consonant, it will be preceded by the article 'a'.Example: a UNIVERSITY, a union, a one rupee note, etc.In the part given in the 2nd option, the article 'a' will be used before the adverb 'valid' as it doesn't have the sound of a vowel. Correct Sentence: Do you have a valid explanation for your absence from work? Any abbreviation having the sound of a vowel will be preceded by the article 'an'.For example:He is a MLA.He is an MLA.