Explanation:

ZnCl2 is a compound that dissociates into Zn2+ ions and Cl- ions when dissolved in water. These ions are responsible for the conductivity of the solution.

Reactivity series:
The reactivity series is a list of metals arranged in order of their reactivity with other substances. It helps us predict how different metals will react with each other. The reactivity series is as follows:

- Potassium (K)
- Sodium (Na)
- Calcium (Ca)
- Magnesium (Mg)
- Aluminum (Al)
- Zinc (Zn)
- Iron (Fe)
- Lead (Pb)
- Hydrogen (H)
- Copper (Cu)
- Silver (Ag)
- Gold (Au)

Explanation for the correct answer:
According to the reactivity series, zinc (Zn) is less reactive than aluminum (Al). This means that zinc can displace aluminum from its compounds.

When zinc chloride (ZnCl2) solution comes into contact with aluminum (Al), a displacement reaction occurs. The zinc ions (Zn2+) in the solution react with the aluminum metal (Al) to form aluminum ions (Al3+) and zinc metal (Zn).

Zn2+ + 2Al → 2Al3+ + Zn

As a result of this reaction, the zinc ions are removed from the solution and are converted back into zinc metal. This reduces the concentration of Zn2+ ions in the solution.

Explanation for the incorrect options:
- Gold (Au): Gold is less reactive than zinc, so it cannot displace zinc from its compounds. Therefore, gold does not react with ZnCl2 solution.
- Lead (Pb): Lead is less reactive than zinc, so it cannot displace zinc from its compounds. Therefore, lead does not react with ZnCl2 solution.
- Silver (Ag): Silver is less reactive than zinc, so it cannot displace zinc from its compounds. Therefore, silver does not react with ZnCl2 solution.

Conclusion:
Based on the reactivity series, the only metal that can displace zinc from ZnCl2 solution is aluminum (Al). Therefore, the ZnCl2 solution cannot be kept in contact with aluminum without losing its concentration.