Step 1: Recall the displacement rule.
A halogen can push out the halide of a halogen below it, because oxidising power falls down the group: $F_2 \gt Cl_2 \gt Br_2 \gt I_2$.
Step 2: Check option (A).
Fluorine is so strong it even oxidises water to oxygen, giving $HF$ and $O_2$. This reaction does occur.
Step 3: Check option (B).
Here chlorine is asked to push fluorine out of $KF$. But fluorine is stronger than chlorine, so chlorine cannot displace it. This reaction does not happen.
Step 4: Check option (C).
Chlorine is above bromine, so it easily displaces bromine from $KBr$. This works.
Step 5: Check option (D).
Bromine is above iodine, so it displaces iodine from $KI$. This works.
Step 6: Pick the impossible one.
Only option (B) breaks the rule, so it is the reaction that does not occur.
\[ \boxed{2KF(aq)+Cl_2(g)\rightarrow 2KCl(aq)+F_2(g)} \]