A. Difference in temperature: 6 degrees.
100(6 / 25) = 24% increase.
B. Difference in temperature: 8 degrees.
100(8 / 32) = 25% increase.
C. Difference in temperature: 9 degrees.
100(9 / 50) = 18% increase.
D. Difference in temperature: 15 degrees.
100(15 / 75) = 20% increase.
25% > 24% > 20% > 18%
Therefore B is the answer.
