Make transformations!
НCl —> MgCl2

2HCl + Mg —> MgCl2 + H2

Explanation:

For HCl to produce MgCl2, it means that metallic Mg must have reacted with the HCl.

The equation for the reaction of HCl with Mg to produce MgCl2 is given below:

HCl + Mg —> MgCl2 + H2

Now we must balance the equation. This is illustrated below:

There are 2 atoms of Cl on the right side and 1 atom on the left side. It can be balance by putting 2 in front of HCl as shown below:

2HCl + Mg —> MgCl2 + H2

Now the equation is balanced

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metals are good conductor of electricity. it has free electrons on their surface. according to electron pool theory, each atom present in a metal crystal losses all of its valence electron and as a result a pool of free is form. these pool of electrons are free to move and   conduct electricity easily   while making metal as good conductor.

the correct answer is a. the ability of electrons to flow throughout the metal.

if it's not useful, feel free to report for mistake in answer.