Final answer:
A work of art achieves balance when its visual elements are arranged to create a sense of unity. This balance can be both symmetrical, as in Perugino's painting, or asymmetrical, where the elements have equal visual weight.
Step-by-step explanation:
A work of art possesses balance when its visual weights or masses are distributed in such a way that they achieve unity. Balance is the even use of elements throughout a work of art, and it sets the dynamics of a composition. In art, unity conveys a sense of completeness and cohesiveness. For example, the symmetry in Perugino's painting, Christ Giving the Keys of the Kingdom to St. Peter, provides not only balance but also a serene stability and formality. However, balanced art doesn't require perfect symmetry; asymmetrical balance can also achieve visual stability when different parts of an artwork have equal visual weight.