Question

Berenice, it's very late, you______ go immediately.

must
could
should
can

Answer 1

Berenice, it's very late, you Should go immediately.

Answer 2

Berenice, its very late, you should / must go immediately.

(Should ) Se utiliza para dar consejos. . Es una recomendación, no una obligación

(must) Expresan una obligación, algo que hay que hacer.

 You must come here immediately (es una orden)

Por eso podría ser cualquiera de las dos respuestas, segun sea el caso .