Final answer:
Among the listed options, a promise to buy all that the promisor needs of an item without specifying a minimum quantity is the scenario that most likely constitutes legal consideration, because it implies a bargained-for exchange where each party receives something of value.
Step-by-step explanation:
Legal consideration is a critical concept in contract law and refers to something of value that is exchanged between parties in a contract. It is an essential element for a contract to be enforceable. To constitute legal consideration, there must be a bargained-for exchange where each party receives something of value. This can be a benefit to one party or a detriment to the other.
Options a, b, d, and e are generally not considered legal consideration. A promise to make a gift (a) is not consideration because there is no exchange. Promising to refrain from doing an illegal act (b) cannot be consideration because one already has a legal duty not to perform illegal activities. A promise based upon past consideration (d) is not valid because the consideration must be present or future, not something that has already been done. A promise based upon a preexisting duty (e) is also not valid consideration, as one is already legally bound to the duty.
Among the given options, a promise to buy all that the promisor needs of an item without specifying a minimum quantity (c) is most likely to constitute legal consideration. This is because it contains the element of a bargained-for exchange where the promisor will receive an undetermined quantity of goods as needed, and the promisee will receive payment for those goods.