I'd guess a lot of people answering B are adding conditions/assumptions not included in the question.
Killing something doesn't necessarily involve suffering.
Vote “suffering” with the understanding that I’m thinking about unjustified, unnecessary killings and ways of killing
Intelligence ie capcity for self awareness, attachment, terror, etc is an important secondary factor because this affects a type of suffering, ie non-physical
Self-awareness isn’t contingent upon intelligence. Computer programs can be intelligent, but not self-aware. My cat is self-aware, but not intelligent. Self-awareness / our sense of self is more akin to our other perceptions, like sensory perceptions or our perception of emotion.
I think “danger to others” is an import criteria too, but “capacity for suffering” seems most important in this list. Emotions and capacity for pain are independent of intelligence. I don’t doubt that most mammals can feel the same depth and intensity of suffering as I can.