Explanation:

Elemental sulfur is oxidized to sulfate primarily by chemolithotrophic bacteria like *Thiobacillus* species (A), which gain energy from this process. *Pseudomonas* (B) are versatile but not specialized for elemental sulfur oxidation to sulfate. *Nitrobacter* (C) are nitrifying bacteria, oxidizing nitrite to nitrate. *Clostridium* (D) are anaerobic bacteria involved in fermentation or nitrogen fixation, not sulfur oxidation to sulfate. Therefore, (A) is the correct answer.