Plane abc and plane bce be the same plane.

a. must

b. may

c. cannot

The answer is: [B]:  may.

 
Plane ABC and Plane BCE  MAY be the same plane.

Note: We can still determine the answer WITHOUT an illustration.

Explanation:
Note that:  Two (2) points) determine a line; whereas three (3) points determine a plane. 
→ As such, Plane ABC and Plane BCE MAY be the same plane.  This would be the case if "Point E" is on Plane ABC.  {That is to say, if "Point A" is on Plane BCE }.

→This would be the only "circumstance".

→ Hence the answer is: [B]:  may.

Answer choice: [A]: "must" ; is incorrect.  
       It is clearly INCORRECT to say that:
"Plane ABC and Plane BCE "must" be the same plane.
Answer choice: [C]: "cannot" is clearly incorrect.
    As per the example explained above, it is clearly INCORRECT to say that:
   ""Plane ABC and Plane BCE "cannot" be the same plane.