Answer:
A. Sally will win because Paul's attempted assignment would increase the duties to which she agreed.
Step-by-step explanation:
Sally will win because having agreed to mow Paul's yard (which is smaller) for $300, Paul is trying to increase her duties while maintaining the same price agreed for his small yard.
If the yards were of comparable size Paul will have a chance of winning.
The compromise that could work is if Paul offers to pay Sally a proportional amount (based on the original rate) for the mowing of his grandmother's yard.