All of these are common shapes of bacteria except

 
Choices are:
a. rod b. spiral c. square d. spherical.

There are three most common bacterial cell shapes. These are:
1. cocci or coccus (spherical) (spherical, from the Greek word for seed/berry) ;
2.  bacilli or  bacillus (rod-shaped) (rod-shaped, from the Greek word for staff); and,
3.  spirilla or spiral (twisted) (curved shape, from spiral). 
Therefore the answer is square. Its letter C.