Final answer:
The monstrous and grotesque forms in medieval Gothic architecture, such as gargoyles, conveyed moral lessons and encouraged religious adherence, serving as didactic tools for an illiterate population.
Step-by-step explanation:
The symbolic significance of the monstrous and grotesque forms included in medieval buildings, particularly Gothic cathedrals, points to C. Conveyance of moral lessons. These forms, such as gargoyles, were not only functional as waterspouts protecting the buildings' foundations but also served a didactic purpose. They represented symbols of evil, intended to instill fear into the observers as a means of encouraging adherence to religious doctrines. The gothic art and architecture often included such elements to educate and evoke spiritual reflection among the largely illiterate population of the era.
Gothic cathedrals such as Notre Dame are adorned with these elements that extend beyond their religious function to serve as visual media for instruction. The orientation towards the divine, the transcendence conveyed by architectural design, and the representation of moral dichotomies through both functional and decorative elements signify the complex interweaving of aesthetics, religion, and education in the Gothic period.