Mg metal is placed in hcl and a gas is liberated. this is an example of a

a)physical property
b)chemical property

The answer to your question is: b) Chemical property

Explanation:

Physical property: is when matter changes is physical state but it does not change its composition. For example, evaporation, condensation, sublimation,  solidification, etc.

Chemical property, is when matter changes its composition because the reactants forms products.

In this question we have:

                                         Mg    +       HCl      ⇒      MgCl₂ + H₂↑

In this reaction, there are Mg and HCl, and after the reaction MgCl₂ and H₂ so, there was a change, and this reaction is a chemical change.