Fmri scans of brain areas vital to driving indicate that activity when a driver is attending to conversation. a study that focused video cams on teen drivers found that a of crashes followed driver distraction from other passengers or phones.

I believe the answer is decreasing; majority.
Our brain's capability to maintain attention is limited. So when we use it to focus on things such as conversation, it will take the attention capacity  that needed for driving.
In 2017 alone, the number of accidents that caused by distractions is 37%, which is considered a majoirty compared to other causes

Ibelieve that the answer is (a).