Answer
$m^2$.
Work Step by Step
Usually, to measure a tarpaulin, we use the area unit. The measurements $m^3$, L, and mL refer to volume, so they would not apply. The measurements ha, $m^2$ and $cm^2 $ refer to the area, but for a truck, ha would be too big and $cm^2 $ would be too small. So, the most reasonable unit here is $m^2$.
