Chemotrophs are organisms that obtain energy by the oxidation of electron donors in their environments. These molecules can be organic (chemoorganotrophs) or inorganic (chemolithotrophs). The chemotroph designation is in contrast to phototrophs, which use solar energy. Chemotrophs can be either autotrophic or heterotrophic. Chemotrophs can be found on ocean floors where sunlight cannot reach or above ground, such as the case with iron bacteria. Some examples of chemoautotrophs include sulfur-oxidizing bacteria, nitrogen-fixing bacteria and iron-oxidizing bacteria. Nitrococcus and Nitrobacter are chemoautotrophs. They are nitrifying bacteria.

Chemoautotrophs are organisms that obtain energy by oxidizing inorganic compounds and use this energy to synthesize organic molecules from carbon dioxide. They do not rely on sunlight for energy like photoautotrophs.

The given options are:
A. Nitrococcus
B. Pseudomonas
C. Thiobacillus
D. Nitrobacter

The correct answer is option 'C' - Only A and C.

Explanation:
1. Nitrococcus: Nitrococcus is a genus of bacteria that are chemoautotrophs. They obtain energy by oxidizing nitrite (NO2-) to nitrate (NO3-). This process is called nitrification and is important in the nitrogen cycle.

2. Pseudomonas: Pseudomonas is a genus of bacteria that are known for their metabolic versatility. While some species within this genus can be chemoautotrophs, the statement does not specify which species of Pseudomonas are included as options. Therefore, we cannot definitively say that Pseudomonas is a chemoautotroph based on the given options.

3. Thiobacillus: Thiobacillus is a genus of bacteria that are chemoautotrophs. They obtain energy by oxidizing sulfur compounds, such as elemental sulfur (S0) or sulfide (S2-), to sulfuric acid (H2SO4). This process is important in sulfur cycling.

4. Nitrobacter: Nitrobacter is a genus of bacteria that are chemolithotrophs. They obtain energy by oxidizing nitrite (NO2-) to nitrate (NO3-). While they are not chemoautotrophs, they play a role in the nitrification process by converting nitrite produced by Nitrosomonas (another genus of bacteria) into nitrate.

Therefore, the correct answer is option 'C' - Only A and C. Nitrococcus and Thiobacillus are the bacteria that are chemoautotrophs based on the given options.