The correct word to complete "En poésie, nous parlons de ___" is vers. The complete sentence means "In poetry, we speak of verses."
In French literary terminology, un vers (plural: des vers) refers to a line of poetry, or a verse. Poetry is composed of vers arranged in strophes (stanzas). The study of vers includes elements such as metre, rhyme, rhythm, and syllable count (e.g., an alexandrin has twelve syllables per verse).
The other options are unrelated to poetry:
Vérin is a mechanical jack or actuator, a technical/engineering term.
Verts means "green" (the plural masculine form of the adjective vert).
Vent means "wind," a meteorological term.
Note that vers is a homophone of vert (green, singular) and verre (glass), so context is essential for distinguishing them. In the context of poésie, the word is always vers.
