Answer
Plot $I$
Work Step by Step
When $u$ is set as constant, the $z$ component becomes constant.
Hence, the grid curves must be parallel to $xy$ plane.
When $v$ is set as constant, the $y$ component becomes constant.
Hence, the grid curves must be parallel to $xz$ plane.
Only plot $I$ has pair of grid curves such that one of them is parallel to $xy$ plane and the other is parallel to $xz$ plane.
