The structure of the given sentence as follows: Subject+has/have+V3+Object.
(Active Voice) Subject (Objective Case)+has/have+been+V3+Object (Subjective Case).
(Passive Voice) 'The plan' will be put in place of 'we'.
(Subject becomes object) And 'We' will be put in place of 'The plan'.
(the object becomes subject) 'Have looked at' will be changed into 'Has looked at'.
(To agree with the changed subject and object) Thus the correct sentence is:
The plan has been carefully looked at