Option a is the correct answer. The gradual decline of towns was an important feature of the Gupta period, which lasted from the 4th to the 6th century CE in India. It faced several challenges and problems that led to the decline of urbanization and civilization. Some of the factors that contributed to the decline of towns during the Gupta period are: Invasion of the Hunas (Hephthalites), a nomadic tribe from Central Asia, who attacked and plundered many towns and cities in northern India. The decline of the central authority and the rise of regional powers, who asserted their independence and autonomy from the Gupta Empire. The Gupta rulers faced many rebellions and revolts from their feudatories, governors, and vassals, who challenged their supremacy and legitimacy. The deterioration of the economic and social conditions, which affected the prosperity and stability of the towns and cities. The Gupta rulers imposed heavy taxes and tributes on the people to finance their wars and administration. The people also suffered from famines, epidemics, floods, droughts, and other natural calamities.