(1) Ich esse ihn.
(masculine accusative form of the pronoun)
(2) Er kocht fuer uns.
Must use accusative form and the only one in that form among the choices is "uns"
(3) Die Speisekarte ist fuer euch.
(same reasoning as with (2) ) The preposition "fuer" alsways needs accusative to follow.
(4) Ich kenne dich!
Accusative form of you.