What is a cluster of polar flagella called?

The correct option is Lophotrichous.

Bacteria can be classified into different groups based on the arrangement and number of flagella (whip-like structures used for locomotion) on their cellular surface:
1. Monotrichous: A single flagellum located at one pole (end) of the cell.
2. Amphitrichous: A single flagellum or a cluster of flagella at both poles of the cell.
3. Lophotrichous: A cluster or tuft of multiple flagella arising from one or both polar ends of the cell.
4. Peritrichous: Flagella distributed randomly all over the entire surface of the cell.

Therefore, a cluster of polar flagella is specifically termed lophotrichous.