The greatest possible surface area of the prism is 78 square units, it is obtained by removing center cube on each side of the prism.
Step-by-step explanation:
The given is,
                 Prism is made up of 27 identical  cubes
Step:1
                Ref attachment,
                Let, surface area of one of cube = 1 square units
                Surface area of given prism,
                In the given diagram it have 9 cube sides in each side of prism.
                                   1 surface prism = 9 surface of cube
                Surface area of given prism = 6 × Surface of prism
                                                               = 6 × 9
                                                               = 54 square units           
Step:2 Check for alternative's
            For removing one cube on the edge of prism,
                       1 surface of prim = 9 surfaces of cube
           Surface area after removing cube on each side,
                                              = 6 × 11 = 66 square units
            For removing  cube on corner of prism,
                       1 surface of prim = 9 surfaces of cube
            Surface area after removing cube on corner,
                                              = 6 × 9 = 54 square units
            For removing center cube on each side,
                      1 surface of prim = 13 surfaces of cube
            Surface area after removing cube on corer on the prism,
                                                   = 6 × 13 = 78 square units
    Surface area after removing corner cube on prism = 78 square units
Result:
        The greatest possible surface area of the prism is 78 square units, it is obtained by removing center cube on each side of the prism.