Final answer:
Diminishing marginal utility starts when each additional unit consumed provides less additional satisfaction than the previous one, exemplified by how less utility is derived from successive T-shirts purchased.
Step-by-step explanation:
Diminishing marginal utility begins to occur as soon as additional units of a given good are consumed and each unit provides less and less additional utility compared to the previous one. For instance, when José purchases his first T-shirt, he gains a significant amount of satisfaction (measured in utils), because it's his favorite. However, the fourth T-shirt he picks has a reduced additional utility since it is only to be used when his other clothes are unavailable, which exemplifies diminishing marginal utility. This concept is an indicator of the law of diminishing marginal utility, which states that the added satisfaction from consuming additional units of a good decreases with each additional unit consumed. This principle of diminishing utility aligns with the broader economic concept of the law of diminishing returns, often discussed in the context of production where the addition of factors of production results in smaller increments in output.