Answer:Unilateral contract
Step-by-step explanation:
Unilateral contract is a type of agreement or contract in which offer is made and fulfilled through performance .An offeror offers any promise to be fulfilled by him/her if the other party completes the performance act successfully.
According to the question,situation is described as unilateral contract as Sarah is offering $150 to Allison for painting her house in exchange.Allison has to complete house painting to gain promised money even though there is no legal contract between them.