Past year sep/dec 2017 Q4 a)

For the option not to be exercised, the futures price would have to go below 94.25. The futures price will be 100 - interest rate - 0.34 (unexpired basis). For this to be equal to 94.25, the interest rate has to be 5.41.

The whole point of considering options is that although they limit the minimum interest rate they allow the company to get the benefit of higher interest rates. If interest rates go above 5.41% then they would not exercise the option and so would get the benefit of the higher rate, whereas futures fix the effective rate whatever happens to the interest rate.