Cerrá la ventana.

English Translation

Close the window.

I do not recognize this verb conjugation. Where does it come from? I normally would have chosen cierra, cierre, cierren, or cerrad for this cloze.

“Cerrá” is the “voseo” form of the imperative, i.e. for using “vos” instead of “tú” in some dialects,
while “cierra” is the normal “tú” form.

Look at the conjugation table on wiktionary:
cerrar - Wiktionary, the free dictionary

and the explanation of voseo:
voseo - Wiktionary, the free dictionary
“in Spanish dialects, the use of the pronoun vos as opposed to