When did L. L. Zamenhof first publish "Unua Libro", the first publication describing his international language Esperanto?

The story behind the answer

1887 is correct because that’s the year L. L. Zamenhof published Unua Libro in Warsaw under the pen name “Dr. Esperanto” (“one who hopes”), introducing the grammar and vocabulary of his planned international language. The first edition appeared in Russian, with Polish, German, and French versions following later that same year.

A handy way to remember: 1887 marks the “birth” of Esperanto;