It’s a unification. The rule for using “a” or “an” is that we use “a” before a consonant sound and “an” before a vowel sound. The word “unification” starts with a consonant sound since its first letter (u) sounds like “you”.

It’s a united. Use "a" before words which sound like they start with a consonant, even if the first letter is a vowel. The word united begins with a consonant sound (you), so it takes the article “a”.

It’s a union. We use “a” with nouns that start with a consonant sound. Sometimes, as in the case of “union”, a word begins with the vowel “u” in the spelling, but with a consonant in the pronunciation (you).

It’s a unicorn. The word “unicorn” begins with a vowel in the spelling, but it makes a consonant sound (a "you" sound).

It’s a one. When deciding whether to use “a” or “an”, you want to choose based on the sound the word begins with, not the actual letter it begins with. Since “one” starts with a “w” sound, “a one” is the right one.

It’s a European. The rule for using “a” or “an” is based on pronunciation of the word that follows, not the spelling. The first sound of the word European (and Europe) is consonant (you).

“A euro” is the correct one. The rule for using “a” or “an” is based on pronunciation, not spelling. Since “euro” begins with a consonant sound (it is pronounced “youroh”), we use the indefinite article “a”.

It’s a euphoric state. The words euphoric and euphoria begin with a “you” sound, not a vowel sound.

It’s “a euphony”. Use “a” (instead of “an”) if the following word starts with a consonant sound. The word euphony, despite starting with a vowel, makes a “you” sound.