The correct answer is: C. those who produce above standard should receive higher wages than those producing below standard.

Taylor's differential piecework plan is a system of wage payment in which workers are paid a higher rate for each unit of output produced above a certain standard. This system is designed to encourage workers to produce more and to reward them for their increased productivity.

Option A is incorrect because Taylor's differential piecework plan does not require that all laborers be assigned work according to their capacity. In fact, the plan can be used in a variety of settings, including those where workers are not all equally skilled or experienced.

Option B is incorrect because Taylor's differential piecework plan does not require that all laborers be put in different shifts. In fact, the plan can be used in a variety of settings, including those where workers are all on the same shift.

Option D is incorrect because Taylor's differential piecework plan does not require that payment be the same on a fixed standard. In fact, the plan is designed to reward workers for their increased productivity, so payment is not the same for all workers.