Select the appropriate article to complete the following sentence. Choose…

Concept: In English, a fixed pair of contrasting nouns or adjectives joined directly by "and" -- with no article before either word -- is a recognised idiomatic pattern for referring to a class or category as a whole rather than to specific individuals. Standard grammar references list this "paired words" pattern under the rules for omitting the article, alongside examples such as "day and night" and "man and wife".

Application: In this sentence, "rich and poor" and "young and old" are exactly such fixed contrasting pairs. They are used to mean "all people, regardless of wealth or age" rather than any one specific rich person or poor person. This is the standard textbook illustration of that paired-words, zero-article pattern, so no article is required before "rich" -- the blank takes the zero article.

Cross-check -- why the other options do not fit:

• "a" would require a singular countable noun starting with a consonant sound right after it, but "rich" here functions as a class-denoting term inside a fixed paired construction, not a single countable noun -- "a rich" does not work in this construction.

• "an" would require a singular countable noun starting with a vowel sound, but the same mismatch applies: "rich" is not being used as a vowel-initial singular countable noun here.

• "the" can combine with a single adjective elsewhere to name a class (e.g. "the poor need support"), but that is a different construction from a fixed contrasting pair joined directly by "and" with no article anywhere before either term -- the sentence here has no article before "rich", "poor", "young", or "old" at any point, which is the marker of the paired zero-article idiom rather than of a "the + adjective" class noun.

Result: The zero article correctly completes the sentence -- "Rich and poor, young and old met together" -- matching the standard idiom for such paired contrasting terms.