If you were to make a physical model of the carbon cycle, what shape would the model

C. Dip a piece of red litmus paper in each solution

Explanation:

In case you have a solution of HCl 1M and tap water, for the second case, it´s probably that pH will be in a around 7 (and a bit above 7,this is, a bit alkaline)

A red litmus paper is a piece (or strip) of paper, which may help to determine the pH of a solution: it will turn into blue color when it is dipped in a alkaline solution (this is, with a pH > 7) or will turn into red colour when it is dipped in an acid solution (this is, with pH < 7)

If using copper, this metal has no reaction to an acid like HCl, since copper has a normal reduction potential higher that hydrogen (from HCl), for which it is difficult to the H⁺ to “steal” electrons from Copper (this is, both Cu and H⁺ will not easily react)

Lastly, when dropping a piece of magnesium into water and HCL, this metal will react as following:

Mg⁰ + 2H₂O → Mg(OH)₂(ac) + H₂  (g)

Mg⁰ + HCl → MgCl₂(ac) + H₂(g)

So both water and HCL will produce a gas (H₂) and this will not help us to differentiate them between each other

So, the best and quick method to distinguish tap water from HCl is with a red litmus paper