## Core Concept
The question tests the understanding of microbiological indicators used to assess water quality, specifically for detecting fecal contamination. Fecal contamination indicators help in evaluating the presence of pathogens in water.

## Why the Correct Answer is Right
The correct answer, **B. E. coli**, is a specific indicator for recent fecal contamination of water because *E. coli* is a gram-negative, rod-shaped bacterium commonly found in the intestines of humans and warm-blooded animals. Its presence in water indicates recent contamination with fecal material. *E. coli* is widely used as an indicator organism due to its ease of isolation, quantification, and its correlation with the presence of pathogens.

## Why Each Wrong Option is Incorrect
* **Option A:** is incorrect because while *Coliform* bacteria are indicators of water contamination, they are not as specific to recent fecal contamination as *E. coli*. *Coliform* bacteria have a broader range of sources, including environmental ones.
* **Option C:** is incorrect because *Clostridium perfringens* is used as an indicator of past fecal contamination. It forms highly resistant spores that can survive longer in the environment, making it less indicative of recent contamination.
* **Option D:** is incorrect because *Streptococcus fecalis* (now known as *Enterococcus faecalis*), while it can be an indicator of fecal contamination, is less commonly used for recent fecal contamination compared to *E. coli*.

## Clinical Pearl / High-Yield Fact
A key point to remember is that *E. coli* is a preferred indicator for recent fecal contamination due to its specificity and relatively short survival time outside the host, making it a practical choice for assessing the immediate risk of fecal pathogens in water.

## Correct Answer: B. E. coli.