Which of the following is an example of a subtopic in a topic cluster?

A subtopic in a topic cluster is typically a focused and specific angle that supports or expands upon a broader main topic. In this case, "Climate change effects on agriculture" fits perfectly as it is directly related to the overarching theme of "Global warming."

This choice narrows down the broad concept of global warming to examine its specific impacts on agriculture, demonstrating how the two concepts are interconnected. Subtopics serve to provide more detailed information that can enhance the reader's understanding of the primary topic by delving into specific areas of interest.

The other options present broader themes or methodologies that, while related to environmental issues, do not specifically delve into a particular aspect of global warming in the same way. "Global warming" itself is too broad to be considered a subtopic, and topics like "Environmental conservation strategies" and "Scientific research methodologies" are too far removed from the specific effects on agriculture to function as subtopics within that particular cluster.