From the low grade copper ore, leached copper can be reduced by either zinc or iron but iron is given preference over zinc because :

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In the hydrometallurgical extraction of copper from low-grade ores, leached copper ions ( $C u^{2 +}$ ) are reduced to metallic copper. This process, known as cementation, can be achieved using a more reactive metal. Both zinc and iron are more reactive than copper and can reduce $C u^{2 +}$ ions.

The relevant reduction reactions are:

1. Using zinc: $C u^{2 +} (a q) + Z n (s) \to C u (s) + Z n^{2 +} (a q)$

2. Using iron: $C u^{2 +} (a q) + F e (s) \to C u (s) + F e^{2 +} (a q)$

While both metals are effective, iron is preferred over zinc primarily due to economic reasons. Iron is a significantly less expensive metal than zinc. Using a cheaper reducing agent helps to minimize the overall cost of copper extraction, making the process more economically viable for low-grade ores.

Therefore, the preference for iron over zinc is due to its lower cost.

The final answer is $B$

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