Economic activities are grouped into sectors. The primary sector draws goods directly from nature (farming, fishing, mining), the secondary sector transforms raw materials into finished goods (manufacturing, construction), and the tertiary sector provides services that support the other two.

Retail trade does not produce or process any good; it buys finished goods from wholesalers and sells them to final consumers. Buying, selling, storing and distributing goods are services, so retailing belongs to the service sector, along with transport, banking and communication.

The primary sector is wrong because a shopkeeper extracts nothing from nature. The secondary sector is wrong because no manufacturing takes place in a shop. The quaternary sector covers knowledge-based activities such as research, information technology and consultancy, not the selling of goods.

Tertiary sector option is correct.