Which type of reaction is mg + s → mgs?

a. polymerization
b. synthesis
c. replacement
d. decomposition

i think it's synthesis, but idk

The type  of reaction that is  Mg +S→  MgS  is a  synthesis reaction

  Explanation

Synthesis reaction  as refers to as direct combination is a reaction  in which two or   more chemical   species   combine  to form   more  complex  product.

 The reaction  of  Mg  +S → MgS  is a synthesis  reaction because;

    Mg combine with  S to form a more complex product MgS