The universal language of maps can be understood through

Concept: A map conveys spatial information through graphic marks rather than through the words of any spoken or written language. Maps commonly provide a legend — a key of colours, lines, hachures, and pictorial icons in which one mark stands for one specific feature (a river, a temple, a railway line, a marsh). Because that legend is a picture-based code rather than a piece of language, any reader who has learned it can decode the map without needing to know the language its place-names are written in. This is exactly what is meant by calling the notation of conventional symbols the “universal language” of a map — a code whose meaning does not depend on the reader's spoken language.

Application: For this question, “universal language of the map” means a way of reading a map that works for every reader once the legend is known, independent of the language that reader speaks. Among the four choices, only conventional symbols meet that description: once a reader has learned what a given legend's marks for “well”, “water”, or “highway” stand for, that reader can decode any map using the same legend, because the meaning is carried by the fixed graphic code, not by written words in a particular language.

• Alphabets — tied to a specific script and language; a reader who cannot read that script gets no information at all, so alphabets cannot be a universal code.

• Thematic maps — a category of map built around one chosen subject (rainfall, population, crops); choosing a theme narrows WHAT is shown, it does not create a shared notation for HOW it is shown.

• Sketch — a freehand, unstandardized drawing; two people sketching the same area produce different marks, so a sketch carries no fixed, agreed meaning that a stranger can decode.

Cross-check: Most printed maps carry their own legend listing the conventional symbols they use; because that legend — not any spoken or written language — is what a reader must learn, the same picture-based code lets any reader of that legend decode the map. This confirms that conventional symbols (not alphabets, a map's theme, or a sketch) are what make a map's language universal.