Given averages 4, 7, 10, 13,...., 28, 31.

The difference between any two numbers written in continuos sequence is 3.

we know the formula

The average of consecutive number =$\frac{\mathrm{First} \mathrm{number}+\mathrm{Last} \mathrm{number}}{2}$

Here, First number= 4 and Last number =31

∴ Required average

$$\begin{gathered} =\frac{4+31}{2} \\ =\frac{35}{2} \\ =17.5 \end{gathered}$$

Hence, Option D is correct.