To summarize what you know so far.
There are two words on the card, of which one is an indefinite article. The answer on the card concerns the written, not the spoken, word. It appears in the vast majority of known languages, it existed in the 19th century, and it is sometimes, though not most frequently, seen in a public space. It could, though need not, be the property of a particular word (subject to the caveat that the words on the card define a property but are not a 'property' as such).
It is not: anarcho-syndicalism, linguistics, a particular language, a saying or phrase, a figure of speech, a G&T (in any language), an alphabet or writing system, related to grammar, punctuation, diacritical characters, metadata, related to pronunciation, an idiom, a diary, a particular word, a cliché, a metaphor, a misspelling, an abbreviation, a heading, cursivity, calligraphy, to do with computer-mediated-communication, a font, an epitaph, a text message, a rebus, a foreignism, a neologism, to do with the law, a label, or a signature.
Reading that through, the only thing that is perhaps misleading is the 'phrase or saying' answer. Just as the answer could (but need not) be a word, so it could also (but need not) be a phrase.