This sentence has an error by lacking an article before a quantitative adjective (one and half). There are two halves in one so an indefinite article must take place before the word 'half'. Option “One and half million barrels of oil flows every” has no article in it. Option “One and an half million barrels of oil flows every” uses an indefinite article AN that is used before a word that starts with vowel sound. Option “One and the half million barrels of oil flows every” uses the definite article (the). Option “One half million barrels of oil flows every” uses an incorrect form of adjective (One half). Option “One and a half million barrels of oil flows every” uses the correct indefinite article that is used before the word that starts with a consonant sound; hence, it is the correct answer.