Final answer:
The relationship where the total number of widgets production increases while the per worker contribution decreases is described by the Law of Diminishing Marginal Product, a result of fixed capital constraints in production.
Step-by-step explanation:
The concept that explains the relationship between the rising total number of widgets produced and the simultaneous decrease in the additional contribution by each worker is A) Diminishing. This is referred to as the Law of Diminishing Marginal Product, which describes that in the short run, as more workers are hired, each additional worker typically contributes less additional output than the previous worker because of the limitations of fixed capital. This phenomenon is part of the broader concept of diminishing marginal returns. The marginal product of workers initially may rise when they are few, but as more workers are added, the fixed capital (equipment, space, etc.) cannot be increased at the same rate, leading to each worker adding less to the total output.