Decreasing of freezing point is a colligative property, which depends on the number of solute particles in the solution. As the number of solute particle increases the freezing point decreases.
Salt water has a lower freezing point than plain water.
Salt water does not have a different mass than plain water. Option A is incorrect.
Salt water does not affect the buoyancy or the mineral dissolved in it. Both option C and D are incorrect
It is called exothermic reaction because it releases heat and light and it is called combustion reaction because it is reacting and is being oxidised by O2 to MgO.
It can also be called as oxidation reaction since Mg is oxidised to MgO.
