In the last part of the question, an equation of a plane is provided. Can the equation $r=pi+qj$ correspond to a 3D-plane? What are its direction vectors?
In my opinion it only has only one direction vector $(0,0,1)$ (this is actually used in the marking scheme of the question) making it a line instead of a plane.
$\bf i$ and $\bf j$ are vectors in space. $\bf i = \vec {(1,0,0)}$ is a unit vector along the $x$ axis and $\bf j$ is a unit vector along the $y$ axis. In the last part of the question $\bf r$ is the $xy$ plane. $p$ and $q$ are allowed to range over $\Bbb R$. I greatly dislike the reuse of $\bf r$ for many things in this question.