Final answer:
Magnesium (Mg) is more reactive than zinc according to the activity series, thus it can replace zinc in a single replacement reaction. Other elements listed like cobalt (Co), chromium (Cr), and gold (Au) cannot replace zinc as they are less reactive.
Step-by-step explanation:
The elements that can replace zinc in a single replacement reaction are typically more reactive than zinc on the activity series. Looking at the activity series, we can determine that magnesium (Mg) is more reactive than zinc, and therefore it can replace zinc in such a reaction. The elements cobalt (Co), chromium (Cr), and gold (Au) are not more reactive than zinc according to the activity series, so they cannot replace zinc in a single replacement reaction.
For example:
3Zn(s) + 2Cr³⁺ (aq) → 3Zn²⁺ (aq) + 2Cr(s)
This reaction shows chromium being reduced and zinc being oxidized, which only occurs because chromium is higher on the activity series, meaning it is more reactive and can replace zinc in the reaction.