It's a run on. It is easy to fix.
Jackson is a very smart little boy. He can count to twenty and is only two years old.
There is an envelope marked with the instructions that it is not to be opened until the death or disappearance of Dr. Jekyll. This implies that Dr. Lanyon knew something secret, but was unwilling to reveal it at first. He waited until there was no more "hope" of Dr. Jekyll staying normal.
The correct answer is C. Setting.
The theme is the underlying idea of the story, the plot is how the events happen, while the language used is self explanatory.
Sit means to be seated.
Set means to place something somewhere.
