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

Leached copper from low-grade copper ore may be reduced by either zinc or iron, although iron is preferred over zinc since it is a less expensive metal.

Because zinc is more reactive than iron and copper, it is more suited for decreasing the leached copper ore. It thus quickly replaced copper. However, as zinc is a more expensive metal than iron, it will be wise and profitable to use iron scrapes.

Hence Correct option is B