The correct answer is C. Hun invasion.
The Gupta Empire was a major Indian empire that ruled over much of the Indian subcontinent from the early 4th to the late 6th centuries. The empire was founded by Chandragupta I in 320 CE, and reached its peak under the reign of his grandson, Samudragupta (335-375 CE). The Gupta Empire was a golden age of Indian culture, and is known for its achievements in art, literature, science, and mathematics.
The Gupta Empire began to decline in the late 5th century, due to a number of factors, including the Hun invasion. The Huns were a nomadic people from Central Asia who invaded India in the early 5th century. They were a fierce and warlike people, and they caused widespread destruction in their wake. The Gupta Empire was unable to withstand the Hun invasion, and it collapsed in the late 5th century.
The other options are incorrect. The Chalukyas were a dynasty that ruled over parts of southern India from the 6th to the 12th centuries. The Greeks invaded India in the 4th century BCE, but they were eventually defeated by the Mauryan Empire. The Pallavas were a dynasty that ruled over parts of southern India from the 3rd to the 9th centuries.
The Hun invasion was a major factor in the decline of the Gupta Empire. The Huns were a fierce and warlike people, and they caused widespread destruction in their wake. The Gupta Empire was unable to withstand the Hun invasion, and it collapsed in the late 5th century.