c
$A$ is orthogonal matrix
$\therefore 4{q^2} + {r^2} = {p^2} + {q^2} + {r^2} = 1\,\,\,\,\,\,.......\left( 1 \right)$
${p^2} - {q^2} - {r^2} = 0\,\,\,\,\,\,...\left( 2 \right)$
and $2{q^2} - {r^2} = 0\,\,\,\,\,....\left( 3 \right)$
Solving $(1),(2)$ and $(3)$
${p^2} = \frac{1}{2}$
$\left| p \right| = \frac{1}{{\sqrt 2 }}$