**Core Concept**
The purification of water involves various methods to remove contaminants, including bacteria, viruses, and other microorganisms. Water quality assessment is crucial to ensure the safety of drinking water. Coliform bacteria, such as E. coli, are commonly used as indicators of water contamination.

**Why the Correct Answer is Right**
The presence of clostridial spores in water does not necessarily indicate recent contamination. Clostridial spores are highly resistant to disinfection and can persist in water for extended periods. They are often used as indicators of past contamination or as a measure of the effectiveness of water treatment processes. In contrast, coliform bacteria, such as E. coli, are more sensitive to disinfection and are commonly used as indicators of recent contamination.

**Why Each Wrong Option is Incorrect**
**Option A:** Coliforms must not be detectable in any 100 ml sample of drinking water. This statement is true, as the presence of coliform bacteria in drinking water is a common indicator of contamination.
**Option C:** Sodium thiosulphate is used to neutralize chlorine. This statement is also true, as sodium thiosulphate is commonly used to neutralize chlorine in water samples, allowing for the detection of bacteria that would otherwise be inhibited by the chlorine.
**Option D:** Coliforms may be detected by multiple tube method and indole production at 44 degrees. This statement is true, as the multiple tube method is a common technique used to detect coliform bacteria, and indole production at 44°C is a characteristic of E. coli.

**Clinical Pearl / High-Yield Fact**
It is essential to note that water purification methods, such as boiling and disinfection, can be effective against most bacteria and viruses, but may not eliminate all contaminants, including clostridial spores.

**✓ Correct Answer:** B. Presence of clostridial spores indicate recent contamination