Final answer:
The Gupta Empire faced internal challenges and external threats from the Huna tribe, leading to its decline.
Step-by-step explanation:
The empire that struggled with a series of incompetent emperors who had difficulties collecting taxes and warding off invasions from the Huna tribe is the B) Gupta Empire. During its later stages, the Gupta Empire faced significant challenges from the Hephthalites, also known as the White Huns, who eventually overran the remnants of the empire in the fifth century. This period of decline was marked by internal weaknesses, including the inability of emperors to effectively manage the empire's finances and to defend its borders from invaders such as the Huna.