If efgh is a parallelogram, then a. it must be a rhombus b. it might be a rhombus c. it cannot be a rhombus

Some parallelograms are also a rhombus.
So that's why the answer is choice B. 

Note: Any rhombus is a parallelogram, but the other way around isn't true. For example, a rectangle that isn't a square is also not a rhombus. Any rectangle is a parallelogram. Not all parallelograms are rectangles.