How can we increase the rate of collisions between the reactants in this reaction?

mg + 2hcl → mgcl2  + h2

According to the collision theory, we can increase the rate of a reaction by increasing the concentration of reactants. We can also increase the rate of a reaction by increasing temperature to safe levels.

In the reaction mentioned in the question, Mg + 2HCl → MgCl2 + H2, if we increase the amount of magnesium or HCl, the reaction will proceed faster. Similarly, if we increase the temperature, then also the reaction rate will increase.