Correct option is A

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•  London is a European city" is correct because "European" begins with a consonant sound (/j/ as in "you"), not a vowel sound, even though the letter "E" is a vowel.

• The choice "An" would be incorrect because "an" is used before words that start with a vowel sound.

• "The" would imply a specific or unique European city, which isn't the intent of this general statement.

• "No article" would result in a grammatically incomplete sentence.

• So, the correct answer is A.

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