This is a good producer of citric acid

The correct option is Aspergillus.

Citric acid is a weak organic acid that is widely used in the food, beverage, pharmaceutical, and cosmetic industries as a natural preservative, flavoring agent, and acidifier.

While citric acid can be extracted from citrus fruits, industrial-scale production relies almost entirely on microbial fermentation because it is much more efficient and cost-effective. The filamentous fungus Aspergillus niger is the primary and most efficient organism used for this industrial production. Under specific nutrient-limiting conditions (particularly low manganese and high sugar concentrations), Aspergillus niger overflows its tricarboxylic acid (TCA) cycle, leading to the continuous synthesis and excretion of large amounts of citric acid into the fermentation medium.

Here is a brief overview of why the other options are not the primary producers of citric acid:
- Clostridium: A genus of anaerobic bacteria mostly known for producing organic solvents (like acetone and butanol via ABE fermentation) or toxins, rather than citric acid.
- Saccharomyces: A genus of yeasts (such as Saccharomyces cerevisiae) primarily used in brewing, baking, and ethanol production via alcoholic fermentation.
- Pseudomonas: A genus of bacteria known for its metabolic versatility and production of certain biopolymers or pigments, but not utilized for commercial citric acid production.