Final answer:
The commonly used agent for promoting polyploidy is colchicine.
Step-by-step explanation:
The commonly used agent for promoting polyploidy is colchicine. Colchicine is a drug that inhibits cell division and can be used to induce polyploidy in plants. When applied to plant cells, colchicine disrupts the assembly of microtubules, which are necessary for proper chromosome segregation during cell division. By inhibiting cell division, colchicine can lead to the doubling of chromosomes, resulting in polyploidy.