Smallest bacterium among these is
A. Dialister
B. Nitrosomonas
C. Bacillus
D. Spirullium.

Hint: Mycoplasma is the smallest living known cell, that lacks proper cell wall and survives without oxygen, size is approx. 200 to 300nm, it is parasitic bacterium but in given option the smallest bacteria Dialister.

Complete answer:
The dialister is the smallest non fermentative, anaerobic, gram negative rod shaped bacteria and its size is approx. 0.15 to 0.3 micrometer. It show pathogenic effect in various body part of the human beings including lungs, brain and dental root canals
B. Nitrosomonas :- it is gram negative, rod shaped, that show chemoautotrophic of bacterial genus. They are used for bioremediation purposes usually to treat polluted water and water treatment plants and important for nitrogen cycling process and its size is larger then dialister and number of nitrosomonas bacteria size varies in length.
 C. Bacillus :- it represents the genus of bacteria which is gram positive, rod shaped bacteria, bacillus mean rod shape, bacillus species either obligate aerobes or facultative anaerobes, that means have ability to thrive without oxygen. Number of bacteria represented by bacillus due to rod shaped structure and their size also vary.
D. Spirillum:- It represents the spiral shaped structure bacteria which, the two types of spiral cell are as follows: spirillum (rigid with external flagella) and spirochete (flexible and internal flagella). Spirillum are gram negative bacteria and have external amphitrichous or lophotrichous flagella. Their range also varies.

So, the correct answer is A. Dialister.

Note:
Dialister represent the particular type of bacteria which is mainly rod in the shape by rest of the option given in the question represent the genus of bacteria which determine their structure or appearance.