Formal balance is
a. the linear symmetry of an images, so that two halves are mirror images of each other.
b. the aesthetics and stability of an object or structure.
c. the arrangement of the elements so that no one part overpowers or seems heavier.
d. the pleasing composition of parts or objects.

Although all of the answers are more or less correct, the one most correct is a. the linear symmetry of an images, so that two halves are mirror images of each other. Basically, it's when you have a picture in which a thing has its mirror image next to it and you don't know which side is the real one and which is the mirror image. Both are equally important and carry the weight of the composition.

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