Answer:
Option B, Galaxy
Step-by-step explanation:
Planet is the smallest entity among the all given options. There are several planets which rotate around a star and both the star and planet are part of some galaxy for example our solar system comprising of sun and several other planets is a part of milky way galaxy.
A global cluster is the disk (core)portion of a galaxy. It is the region where the density of stars is the highest. Thus global cluster is also a part of galaxy.
Thus among the given options it is the galaxy which is the largest.