Actually two of the answers are correct here. Zinc, silver, copper, and potassium are all metals, so they all will have high melting points. As metals, they will all conduct electricity in either the solid or liquid (molten) state. But the final part of the question is really the key here. Neither copper nor silver will readily react with dilute hydrochloric acid. Zinc, however, will react with dilute HCl to form zinc chloride and hydrogen gas (H2). The equation is given below:
`Zn + 2HCl -gt ZnCl_2 + H_2`
The hydrogen gas will bubble from the solution and will light a flame in a classic lab experiment. Potassium (K) will also react with just plain water (found in dilute HCl) to also form H2:
`2K + 2H_2O -gt 2KOH + H_2`
So both A and D are correct here.