h = 3a + 28.6A pediatrician uses the model above to estimate the height h of a b...

Choice A is correct. In the equation h D + 28.6, if D, the age of the boy, increases by 1, then h becomes h = 3(a+1) + 28.6 = 3a + 3 + 28.6 = (3a + 28.6) + 3. Therefore, the model estimates that the boy's height increases by 3 inches each year.
Alternatively: The height, h, is a linear function of the age, a, of the boy. The coefficient 3 can be interpreted as the rate of change of the function; in this case, the rate of change can be described as a change of 3 inches in height for every additional year in age.
Choices B, C, and D are incorrect and are likely the result of dividing 28.6 by 5, 3, and 2, respectively. The number 28.6 is the estimated height, in inches, of a newborn boy. However, dividing 28.6 by 5, 3, or 2 has no meaning in the context of this question.