'The art of cutting trees and bushes into ornamental shapes' is Option D, Topiary. Option A, 'Horticulture' is art of garden cultivation. Option B, 'Bonsai' is art of growing artificially dwarfed trees and shrubs in pots. Option C, 'Pruning' is cutting branches from the trees or bushes so that they grow better in future. Thus, options A, B and C are incorrect and the correct word for the phrase is Option D, Topiary.