Answer
$A$ is an invertible matrix.
(Th.8, (a) and (c))
Work Step by Step
We see 4 pivot positions in a 4$\times$4 matrix.
So, looking at Th.8,
$\mathrm{c}.\quad A$ has $n$ pivot positions.
is valid. Then,
$\mathrm{a}. \quad A$ is an invertible matrix.
is valid as well
$A$ is an invertible matrix.
(Th.8, (a) and (c))
