Answer: the correct option is B. Commercial Impracticability.
Explanation: Commercial impracticability is the phenomenon in which an event occurs and causes the performance of a contractual duty to be excessively burdensome, unbearably difficult, or extremely expensive, for the party committed to such performance.
The reason for commercial impracticability is due to unforeseen events which did not occur due to a fault or carelessness of either party, and therefore cannot be controlled. After the event has taken place, and it affects the ability of either party to meet the contract, and the parties cannot agree as to the proposed changes in the contract, then the court will be involved in order to determine the commercial practicability of such performance and obligations of the parties involved.
In the scenario given above, we can see that the florist is not to be blamed for the diseases affecting the roses, and therefore, commercial impracticability will be the best line of defense in this case.