The answer is B. Al + O2.
In a synthetic reaction, two (or more) compounds combine and form one complex product. It can be expressed as:
A + B → AB
So, let's go through all of the reactions:
A. 2Li + 2HCl → 2LiCl + H₂
B. 4Al + 3O₂ → 2Al₂O₃
C. Ca + 2H₂O → Ca(OH)₂ + H₂
D. F₂ + 2HCl → 2HF + Cl₂
So, in all of these reactions, there are two products except in the choice B, where only one product. So, the example of synthesis reaction is: 4Al + 3O₂ → 2Al₂O₃.