The correct answer is (d), Cholas.
The tripartite struggle was a power struggle between the Pratiharas, Palas, and Rashtrakutas in northern India from the 8th to the 10th centuries. The Cholas were a powerful dynasty in southern India that did not participate in this struggle.
The Pratiharas were a Rajput dynasty that ruled over much of northern India from the 7th to the 10th centuries. They were the main rivals of the Palas and Rashtrakutas.
The Palas were a Buddhist dynasty that ruled over much of eastern India from the 8th to the 12th centuries. They were the main rivals of the Pratiharas and Rashtrakutas.
The Rashtrakutas were a Hindu dynasty that ruled over much of western India from the 8th to the 10th centuries. They were the main rivals of the Pratiharas and Palas.
The Cholas were a Tamil dynasty that ruled over much of southern India from the 9th to the 13th centuries. They were not involved in the tripartite struggle, but they were a powerful force in their own right.