Explanation

(b)
Given equation of plane is
$2x + 4y - 5z = 10$
$Here,\ a = 2,\ b = 4,\ c = - 5$
Let $OP$ be the perpendicular from $O$ to the plane, then its equation is
$\frac{x - 0}{2} = \frac{y - 0}{4} = \frac{z - 0}{- 5}$
Here, direction ratio are $(2,4, - 5).$
Now, equation of line in vector form is
$\overrightarrow{\mathbf{r}} = 0 + k(2,4, - 5)$
$= (2k,4k, - 5k),\ k \in R$
$\left\lbrack \because equation\ of\ line\ is\ \overrightarrow{\mathbf{r}} = \overrightarrow{\mathbf{a}} + \lambda\ \overrightarrow{\mathbf{b}} \right\rbrack\$