The y-component is proportional to x, so pointing south to the left of the y-axis, and north to the right. The size of this component is proportional to x, so gets larger the further from the y-axis. This eliminates II and IV.
The x-component is constant on circles, eliminating III, and is always positive (except at the origin, where it's zero: hence, all vectors point east, consistent with I.
Additional good remarks:
- Plot II is the only vector field w/o a constant component.
- Looks like the answer to #1, only in 3-D.
- The field must show symmetry about the origin.
- The field should radiate out from the origin.
- The field should stream out from the origin.