Let us first define what is a rising action, and also let us differentiate it from the some of the other elements of the plot.
The plot (backbone, map) of a story or novel begins with the initial incident which, in this particular case, comes in the form of a frame narrative by Robert Walton; a narrative that goes back and forth through letters, and memories.
A seemingly-detached but yet important moment in the storyline as a whole, this very start of the novel relates events, provides a background, but hardly moves the plot forward; "something" must happen to push us into the action, and to move us all the way to the climax. As a result, the inciting or rising action moves in to provide that transition. This is the situation that triggers everything that happens in the novel.
In Frankenstein,it is very clear that the one event that triggers the sequence of incredible twists and turns is the creation of the creature. Had there not been any creature, there would not be a story to tell about it. Hence, this is the rising action that will take us straight to the climax, or point of highest intensity of the story, which is the eventual murder of Elizabeth by the creature.