An act because starting to drive was a positive act that created a risk of harm.

An act because the failure to brake forms part of the ongoing act of driving, rather than a separate omission. 

Neither, because the act/omission distinction makes no practical difference once a duty exists. 

An omission because it is a failure to take a positive step (braking) to prevent harm. 

An omission because the driver merely refrained from acting when the law imposes no general duty to act.