According to the author's description, the word "ebullient" is meant in the sense of "exuberant" in this excerpt.
A. Temperamental would mean that the character described in the text has frequent, unpredictable and impulsive mood changes, going from very positive to very negative. Yet Dreiser writes "always ... radiant," and "for the most part ... spirited and bubbly." This shows that the character's mood is more often on the positive side.
B. Overwhelmed means overcome by emotions, and usually has a negative connotation. Dreiser's depiction of the character is positive, and he probaly would not have used praises like "admiration" or "your ... rages or moods could scarcely withstand his smile" if he meant to describe someone in a less flattering way. An overwhelmed person is also usually low in energy, whereas the character is described as very dynamic ("spirited and bubbly").
C. Exuberant means enthusiastic, joyous, and vigorous. Other synonyms found in the text are "bubbly," "radiant," and "spirited."
D. Frenzied implies an unhealthy amount of emotional disturbance, and this is not the tone chosen by the author.
E. Sympathetic means being sensitive to other people's emotions. However, Dreiser is not describing his character's moods in reaction to other people's.
