Which event foreshadows the resolution of "The Cask of Amontillado" and creates mystery in the story?

Question

Montresor tells Fortunato he has bought a large barrel of amontillado.

Fortunato has a severe cold, which seems to concern Montresor.

Montresor drinks to Fortunato's health with a bottle of Médoc.

Fortunato asks Montresor if he is a Mason.

Answer 1

Montresor tells Fortunato he has bought a large barrel of amontillado.