Which of the following substances has been responsible for naturally occurring radioactive pollution along certain stretches of the coast of Kerala?

Introduction / Context:
This question relates to environmental science and nuclear minerals. Some coastal areas of India, particularly along the Kerala coast, are known for naturally occurring radioactive sands. The question asks which substance is responsible for this radiation, which is important for understanding both environmental issues and nuclear resource distribution.

Given Data / Assumptions:

• The location is the coast of Kerala.
• The pollution mentioned is radioactive in nature.
• Options include plutonium, zinc, thorium and radium.
• We assume knowledge of monazite sands and their composition.

Concept / Approach:
Kerala's coastal sands contain monazite, a mineral rich in thorium. Thorium is a naturally occurring radioactive element, and its presence in beach sands leads to background radiation levels that are higher than average. Plutonium is a man-made transuranic element used in nuclear weapons and reactors, not naturally concentrated in Kerala sands. Zinc is a non-radioactive metal in this context, and radium, while radioactive, is not the primary element associated with the Kerala coast in this way.

Step-by-Step Solution:
Step 1: Recall that Kerala's black sands are known to contain monazite. Step 2: Recognise that monazite is a phosphate mineral bearing rare earths and thorium. Step 3: Understand that thorium is a radioactive element whose presence leads to naturally high background radiation. Step 4: Eliminate plutonium, as it is not naturally found in such coastal deposits and is mostly man-made. Step 5: Exclude zinc and radium, since zinc is not radioactive in this context and radium is not the main element associated with Kerala's monazite sands.

Verification / Alternative check:
Environmental studies and nuclear resource surveys of India repeatedly mention Kerala and parts of Tamil Nadu as containing significant thorium deposits in monazite sands. These areas are examined for thorium-based nuclear fuel cycles. The mention of thorium-rich monazite in Kerala confirms that thorium is responsible for the observed radioactive pollution.

Why Other Options Are Wrong:
Plutonium: A synthetic element produced in nuclear reactors and used in nuclear weapons, not naturally occurring in significant quantities in coastal sands.

Zinc: A common industrial metal that is not naturally radioactive and does not cause radioactive pollution in this context.

Radium: Although radium is radioactive, the coastal sands of Kerala are specifically known for thorium-bearing monazite, not radium deposits.

Common Pitfalls:
Learners may choose plutonium or radium simply because these names are strongly associated with radioactivity. However, the key is to link the location Kerala coast with thorium-bearing monazite sands, which is a standard fact in Indian geography and nuclear resource discussions.

Final Answer:
The naturally occurring radioactive pollution along parts of the Kerala coast is mainly due to Thorium in monazite sands.