Final answer:
The event that might lead to frustration of the contract between MacBeth Homes and the Adams family is the river flooding, which submerges the lot and could make home construction impossible.
Step-by-step explanation:
An event that might amount to frustration of a contract between MacBeth Homes and the Adams family in the given scenario is if the river floods, submerging the lot. Frustration of a contract occurs when an unforeseen event renders the contractual obligations impossible to fulfill, or fundamentally changes the purpose of the contract. In this case, the flooding of the lot where the home is supposed to be built could make it impossible for MacBeth Homes to construct the house, thus potentially leading to a legal frustration of the contract. This is distinct from the Adams family changing their mind, which would be a voluntary alteration, the construction of a bridge improving access, which does not affect the ability to perform the contract, or receiving advance payment, which is a normal part of contract fulfillment.