According to the context of the sentence, for it to be complete and make sense we must add the missing word as it follows:
Según el <u>cártel</u>, la función comienza a las 8 de la noche.
<em>Translation</em>: According to the <u>poster</u>, the show begins at 8 at night.
A poster refers to a sheet that presents some type of information that is intended to be disseminated.
It captures the attention of the viewer, forcing him to perceive the message, his function to announce an event and disseminate information.
In the sentence, the <em>''</em><em>cártel</em><em>''</em> complies with informing the spectators of the start time of the show.
Therefore, we can conclude that the correct missing word in the sentence is<em> cártel (post)</em>, which is planned to give certain information about the show.