Final answer:
The ivy-covered walls are the most prominent feature of the house's exterior described in the text, giving it a picturesque and charming appearance.
Step-by-step explanation:
The most prominent feature of the exterior of the house, as described, is the ivy-covered walls. The text describes the house as 'picturesque' with 'vines growing up the outside stone chimneys' which indicates that the ivy was a significant feature. While aspects like red doors, towering chimneys, and stained glass windows might contribute to the house's overall appearance, the description emphasizes the natural, charming aesthetic provided by the ivy-covered walls, which fits with the romantic and Old World charm of the place.
The most prominent feature of the exterior of the house is the ivy-covered walls. In the given description, it is mentioned that the house has walls mantled in roses and ivy, which suggests that the ivy-covered walls are a prominent and visually striking aspect of the house's exterior. Therefore, option C) Ivy-covered walls is the correct answer.