The correct answer is: A. Au
Zone refining is a method of purifying semiconductor materials and other materials. It works by repeatedly melting a small section of a rod of the material and then allowing it to solidify. The impurities in the material are more likely to be concentrated in the molten section, so they are left behind when the section solidifies. This process can be repeated many times to achieve a high degree of purity.
Gold is a good material for zone refining because it has a high melting point and is relatively easy to melt and solidify. The impurities in gold are typically other metals, such as silver, copper, and platinum. These impurities can be removed by zone refining to produce very pure gold.
The other options are not as good materials for zone refining. Germanium has a lower melting point than gold, so it is more difficult to melt and solidify. Silver and copper are both relatively easy to melt and solidify, but they are not as good at removing impurities as gold. Copper is also a relatively common metal, so it is more difficult to find high-purity copper.