Question

2Mg+O2-->2MgO why the given reaction is a synthesis reaction

Answer 1

Since two compounds react to give a single product, MgO, it is a combination reaction or synthesis reaction as it synthesizes MgO.
Fun fact : Mg is usually taken in form of magnesium ribbon and burnt to produce bright white flame and it forms ashes which is MgO.

Answer 2

here 2 compounds react to give a single compound that's why we call the given reaction as a synthesis reaction. 2Mg + O₂ react to give a single product 2MgO
hope it helps ya