Which of the following is the most pronounced physical change in middle and late childhood?

A) head circumference
B) waist circumference
C) leg length
D) proportional changes in relation to height

Proportional changes in relation to height is the most pronounced physical change in middle and late childhood. During this period, children experience a significant increase in height, which leads to changes in body proportions such as the elongation of limbs and torso, and a decrease in relative head size. Therefore, D is the correct choice as it best describes the most pronounced physical change during this period.