The President of which country attended the first Republic Day celebrations of India as its chief guest?

The correct option is Indonesia.

Step-by-step Explanation:
1. Historical Context: India celebrated its first Republic Day on January 26, 1950. This landmark day marked the adoption of the Constitution of India, officially transitioning the country into a sovereign democratic republic.
2. The Chief Guest: The chief guest for this historic first Republic Day celebration was President Sukarno, the founding President of Indonesia.
3. Diplomatic Significance: The invitation highlighted the close anti-colonial solidarity, shared history, and strong diplomatic ties between India and Indonesia during their early years of independence.