Step-by-step explanation:
area of the cross section = πr²-π(1/2r)²
[tex] = \pi( {r}^{2} - \frac{ {r}^{2} }{4} )[/tex]
[tex] = \pi( \frac{4 {r}^{2} - {r}^{2} }{4}) [/tex]
[tex] = \pi \frac{3 {r}^{2} }{4} [/tex]
option D
