WS$20\mathrm{.}1.$ Loop on $2$D Array: What does it do$?$

How would you accurately describe the variable $x$ in the code segment below?

int data$[,$

$\{0,-9,-5,4\},$

$\{-5,-6,-2,-6\},$

$\{-2,2,7,-1\}$;

int $x=0$;

for $($int index $=0$; index $<mn\; id="EditorElement\_12615989">4</mn>$; index$++)$

index$][$index$]$;

$($a$)$ is the sum of the column at a given row index.

$($b$)$ never changes since the code inside the loop never executes.

$($c$)$ is the sum of the array's diagonal elements.

$($d$)$ is the sum of all the elements in the matrix.

$($e$)$ is the sum of the row at a given column index.

What should be the value of at the end?

Additional attempts available with new variants