**Core Concept**
Presence of Clostridial spores in water indicates past contamination, not recent contamination. Clostridial spores are highly resistant to environmental stressors and can remain viable for extended periods. Their presence suggests that the water source may have been contaminated in the past, but it does not necessarily indicate ongoing contamination.

**Why the Correct Answer is Right**
The presence of Clostridial spores in water is an indicator of past contamination because these spores can survive for years in the environment. They are not typically associated with recent contamination events, as they require specific conditions to germinate and multiply. In contrast, recent contamination is more likely to be indicated by the presence of rapidly multiplying organisms such as coliforms or Escherichia coli (E. coli).

**Why Each Wrong Option is Incorrect**

**Option B:**
Coliforms must not be detectable in any 10 ml sample of drinking water is a true statement. The presence of coliforms in drinking water is a common indicator of fecal contamination and is typically used as a proxy measure for the presence of more pathogenic organisms.

**Option C:**
Sodium thiosulphate is indeed used to neutralize chlorine in water samples. This is because chlorine can inactivate or kill organisms, making it difficult to detect them in water samples. Sodium thiosulphate is used to quench the chlorine, allowing the presence of microorganisms to be accurately assessed.

**Option D:**
Coliforms may be detected by multiple methods, including the indole production test. The indole production test is a specific method used to detect the presence of E. coli, a type of coliform bacteria, by assessing the production of indole from tryptophan.

**Clinical Pearl / High-Yield Fact**
When assessing water quality, it's essential to consider the presence of Clostridial spores as an indicator of past contamination, rather than recent contamination. This is because these spores can remain viable for extended periods and do not necessarily indicate ongoing contamination.

**✓ Correct Answer: A. Presence of Clostridial spores indicate recent contamination of water**