The molecular geometry of phosphorus tribromide (PBr3) is trigonal pyramidal.
This conclusion is based on the VSEPR (Valence Shell Electron Pair Repulsion) theory, which helps us predict the shape of a molecule based on the arrangement of electron pairs around the central atom. In the case of PBr3, phosphorus is the central atom surrounded by three bromine atoms and has one lone pair of electrons. The presence of the lone pair creates a repulsive force, causing the bonding pairs of electrons (the P-Br bonds) to arrange themselves in a way that minimizes this repulsion.
As a result, PBr3 adopts a trigonal pyramidal shape, resembling that of ammonia (NH3), which also has a similar arrangement with a central atom bonded to three other atoms and a lone pair. Therefore, the correct answer is option A: trigonal pyramidal.