The correct answer to this question is pneumonia. Victor Frankenstein contracts this infection when he travels across the cold, icy Arctic Ocean. He was crossing this ocean to escape a monster but instead contracted the illness. Some say that the Monster is the cause of death because that is why Victor Frankenstein put himself in a predicament to be in the ocean.
However, the Monster did not directly kill Victor Frankenstein. Instead, pneumonia did. The irony is that the same thing that he created, the Monster, is what led him to flee, contact pneumonia, and lead him to his death.
The answer given by Wyatt is wrong. Victor Frankenstein in the original books dies while chasing the monster, trying to kill it. He follows it to the Arctic Circle but while chasing it falls through an ice floe and contracts pneumonia. He is rescued but dies after telling his story to the captain of the ship that rescues him. Victor Frankenstein's death is very impactful on his monster after which the monster vows to commit suicide.