(B)

Potassium iodide has the formula KI—a 1:1 molar ratio. However, the mass of iodine is approximately 3 times the mass of potassium, so the iodine has 3 times the contribution to the percent mass.

This problem may also be solved by dividing the molar mass of both potassium and iodine into the total molar mass and then multiplying by 100.