Explanation

Thank you. That was a very nice dinner.

article (a, an) is used before a noun

dinner is a noun

"a" is used before singular, countable nouns that begin with a consonant sound.

"very nice dinner" refers to one specific dinner, but since "nice" starts with a consonant sound, "a" is the correct choice.

"an" is incorrect because it is used before words starting with a vowel sound.

"the" is incorrect because it refers to something specific that the speaker and listener already know about.

ND11-4-2023