Final answer:
Montresor’s true intentions are captured in the line where he mentions fitting the last stone into the wall, highlighting his plan to entomb Fortunato alive.
Step-by-step explanation:
The line from Edgar Allan Poe's The Cask of Amontillado that reveals Montresor’s true intentions in his dealings with Fortunato is option c: "I had finished a portion of the last and the eleventh; there remained but a single stone to be fitted and plastered in." This quote distinctly uncovers Montresor's actual intention, which is to entomb Fortunato alive in the catacombs as part of his revenge. Montresor's meticulous planning and the final act of sealing Fortunato behind the wall signify the culmination of his scheme to punish Fortunato with impunity.