Final answer:
The geometry of the SO3 molecule is best described as trigonal planar.
Step-by-step explanation:
The geometry of the SO3 molecule is best described as trigonal planar. Trigonal planar geometry occurs when there are three electron groups around the central atom, arranged in a flat, triangular shape. In the case of SO3, there are three oxygen atoms surrounding the sulfur atom, giving it a trigonal planar molecular geometry.