**Core Concept**
Water quality is often assessed by its bacteriological safety, which is determined by the presence of specific microorganisms. These indicators are used to evaluate the risk of waterborne diseases.

**Why the Correct Answer is Right**
_Escherichia coli (E. coli)_ is a gram-negative, rod-shaped bacterium commonly found in the human gut and in the environment. It is a well-established indicator of fecal contamination and water quality. The presence of _E. coli_ in water suggests recent fecal pollution, which can pose a risk to human health. _E. coli_ is used as an indicator organism because it is easy to isolate and culture, and its presence is closely correlated with the presence of other pathogens.

**Why Each Wrong Option is Incorrect**
**Option A:** _Pseudomonas aeruginosa_ is a gram-negative bacterium that can be found in various environments, including water. However, it is not typically used as a bacteriological indicator of water quality because its presence does not necessarily indicate fecal contamination.

**Option B:** _Staphylococcus aureus_ is a gram-positive bacterium commonly found on human skin and in the environment. While it can be present in water, it is not a reliable indicator of water quality because its presence does not necessarily indicate fecal contamination.

**Option C:** _Bacillus subtilis_ is a gram-positive bacterium commonly found in soil and water. It is not typically used as a bacteriological indicator of water quality because its presence does not necessarily indicate fecal contamination.

**Clinical Pearl / High-Yield Fact**
When assessing water quality, it is essential to consider multiple indicators, including bacteriological, chemical, and physical parameters. _E. coli_ is a key bacteriological indicator, but other organisms, such as _Clostridium perfringens_ and _Enterococci_, may also be used in specific contexts.

**Correct Answer: D. Escherichia coli**