In which of the following states/union territories was an election NOT held during March-April 2021?

The correct answer is Bihar.

To understand why, let us look at the legislative assembly elections held in India during March-April 2021:
1. West Bengal: Legislative assembly elections were held in 8 phases from March 27 to April 29, 2021.
2. Tamil Nadu: Legislative assembly elections were held in a single phase on April 6, 2021.
3. Puducherry: Legislative assembly elections for the Union Territory were held in a single phase on April 6, 2021.
4. Kerala and Assam: Elections were also held in these states during March-April 2021.

In contrast, the Bihar Legislative Assembly election was held earlier, in three phases from October 28 to November 7, 2020. Therefore, elections were not held in Bihar during March-April 2021.