The robot that steals the plant from EVE aboard the Axiom is GO-4.
GO-4 is the Axiom captain’s first-mate robot and secretly follows AUTO’s orders. After EVE’s plant disappears, WALL-E and EVE discover GO-4 placing it in an escape pod programmed to self-destruct. WALL-E enters the pod to recover the plant, survives its destruction, and returns it to EVE.
The other choices belong to different roles. AUTO is the Axiom’s autopilot and principal antagonist, while M-O is the cleaning robot obsessed with removing contamination from WALL-E. BURN-E is the star of a separate short film included with some WALL-E home-video releases, not the plant thief in the feature film.
GO-4’s theft is part of the film’s larger conflict over directive A113, which forbids the Axiom from returning to Earth. The plant is essential because it proves Earth can support life again.