**Core Concept**
Chlorination of water is a widely used disinfection method to eliminate pathogens and microorganisms, ensuring the safety of drinking water. The process involves the addition of chlorine to water, which then reacts with the water to form hypochlorous acid (HOCl) and hypochlorite ions (OCl-). These compounds have antimicrobial properties, effectively killing bacteria, viruses, and other pathogens.

**Why the Correct Answer is Right**
The correct answer is based on the understanding that chlorination of water is effective against a broad spectrum of microorganisms due to the formation of hypochlorous acid and hypochlorite ions. These compounds work by denaturing proteins, disrupting cell membranes, and interfering with the metabolic processes of microorganisms, ultimately leading to their death. The effectiveness of chlorination in water treatment is well-documented, and it is widely recognized as a crucial step in ensuring the safety of drinking water.

**Why Each Wrong Option is Incorrect**

**Option A:** Chlorination of water does not effectively eliminate protozoa such as Giardia and Cryptosporidium.
Chlorine is not effective against these protozoa, and other methods like filtration or UV treatment are required to remove them from water.

**Option B:** Chlorination of water increases the risk of cancer.
There is no conclusive evidence to support this claim. In fact, the US Environmental Protection Agency (EPA) has established a maximum allowable level of chlorine in drinking water to minimize potential health risks.

**Option C:** Chlorination of water is not effective against bacteria such as E. coli.
Chlorination is actually effective against E. coli and other bacteria, making it a crucial step in water treatment.

**Option D:** Chlorination of water requires a high pH level to be effective.
Chlorination is effective over a wide pH range, typically between 4.5 and 9.5. A high pH level is not required for chlorination to be effective.

**Clinical Pearl / High-Yield Fact**
Chlorination of water is a critical step in ensuring the safety of drinking water, but it is not a foolproof method. Other methods like filtration, sedimentation, and UV treatment are also used in combination with chlorination to provide a comprehensive approach to water treatment.

**Correct Answer:** D.