**Core Concept**
The proposed guideline value for radioactivity in drinking water is a critical public health parameter that ensures the safety of water supplies from radionuclide contamination. This concept is rooted in the field of environmental health, specifically in radiation protection and nuclear medicine.

**Why the Correct Answer is Right**
The World Health Organization (WHO) has established a guideline value for radioactivity in drinking water, which is 0.1 Bq/L for gross alpha activity and 1 Bq/L for gross beta activity. These values are based on the risk of cancer from exposure to radionuclides in drinking water. The guideline values are set to minimize the risk of cancer from radionuclide ingestion, while also taking into account the practicality of implementing these values in water treatment and distribution systems.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not provide a specific value for radioactivity in drinking water.
**Option B:** This option is incorrect because it is not the correct guideline value for radioactivity in drinking water as established by the WHO.
**Option C:** This option is incorrect because it is not a valid or established guideline value for radioactivity in drinking water.

**Clinical Pearl / High-Yield Fact**
It is essential for healthcare professionals, particularly those involved in environmental health and radiation protection, to be aware of the guideline values for radioactivity in drinking water to ensure public health and safety.

**Correct Answer: B. 1 Bq/L for gross beta activity.**