(B) Nature of activities
Primary, secondary, and tertiary sectors are classified on the basis of the nature of activities — whether activities involve natural resource extraction, manufacturing, or services.
Source: Let's Recall, Chapter 2
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The textbook explicitly states: "In dividing the economic activities into three sectors — primary, secondary, tertiary — the criterion used was the 'nature of activity'." Note that ownership is the basis for public/private sector division, and employment conditions are used for organised/unorganised sectors — common distractors in this MCQ.