We are looking to weaken the argument that the government Is at fault for the rise in killings because of their failure to stop dangerous knives from being sold- as these knives are a major factor in the majority of killings which are unplanned attacks in the family.
(A) only relates to planned murders, and the argument is about unplanned murders. (B) does not really impact the argument at all, as the argument is unconcerned with attacks that do not escalate to homocide (C) does not directly undermine the argument, and might actually strengthen it as it shows that these knives are a factor in other muders (D) mischaracterizes the argument, as the argument does not discuss the intent of the buyer when they bought the knife (E) could plausibly weaken the argument. It allows for the possibility that these knives are just ordinary kitchen knives, which were already common before the spike. Since this would mean that the government was not responsible for the spike as these knives were already common, and in the past (when they were already common) homocide rates were lower.
While E feels somewhat tenuous, it is just plausible enough to weaken the argument, and it is by far the "least bad" answer.