Groups and the Group Axioms · Challenge problem

Exactly one of these four operations on $S=\mathbb{Q}\setminus\{0\}$ makes $(S,*)$ a group: (A) $a*b=ab$; (B) $a*b=a/b$; (C) $a*b=a+b$; (D) $a*b=a^2b$. Which one is it?