The Shailendra kings who had close contacts with the Indian rulers were followers of which of the following religions?

The correct option is Buddhism.

Explanation:
The Shailendra dynasty ruled parts of Java and Sumatra in Southeast Asia starting around the 8th century. The rulers of this dynasty were prominent followers and grand patrons of Mahayana Buddhism, and they maintained close religious, cultural, and diplomatic ties with various contemporary Indian rulers.

Key Historical Evidence:

1. Connection with the Pala Dynasty (Nalanda):
The Shailendra king Balaputradeva established a monastery for Buddhist pilgrims and scholars at Nalanda in Bihar, India. At his request, the Pala ruler Devapala granted five villages for the upkeep of the monastery and its resident monks, as recorded in the famous Nalanda Copper Plate Inscription.

2. Connection with the Chola Dynasty (Nagapattinam):
The Shailendra ruler Sri Mara-Vijayottungavarman built the Chudamani Vihara, a Buddhist monastery at Nagapattinam in Tamil Nadu. The Chola king Rajaraja I granted the revenue of a village to support this monastery, a patronage documented in the Leiden Plates.

3. Architectural Monuments:
The Shailendra kings built several magnificent Buddhist monuments in Southeast Asia, the most famous of which is the colossal Borobudur temple complex in Central Java, recognized as one of the world's greatest Buddhist monuments.

Thus, the Shailendra kings were followers of Buddhism.