Which of the following best describes the radiation pattern of a quarter-wave ground-plane vertical antenna?
Why A vertical monopole over a ground plane is symmetrical around its vertical axis, so it radiates equally well in every compass direction. The pattern only varies with elevation angle: strongest at low angles toward the horizon, with a null straight up off the end of the radiator. That azimuthal symmetry is exactly what "omnidirectional in azimuth" means, and it is why verticals are popular for DX and for mobile use where you cannot rotate an antenna.
Watch out Isotropic and hemispherical both imply equal radiation in all directions including straight overhead, but no real antenna is isotropic and a vertical has a deep null at the zenith. Bi-directional in azimuth describes a horizontal dipole, which has two main lobes broadside to the wire.
Vertical = all compass directions, nothing straight up. Horizontal dipole = two lobes broadside.