(b) Secondary product
Natural products being changed into other forms through manufacturing is an activity of the secondary sector, so the product is called a secondary product.
Source: Sectors of Economic Activities, Chapter 2
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The textbook states: "The secondary sector covers activities in which natural products are changed into other forms through ways of manufacturing." Examiners expect you to directly link the transformation of natural products to the secondary sector. Options (c) and (d) — tertiary and quaternary — are distractors; tertiary refers to services, not manufactured goods.