The part of the brain that is being tested when a police officer asks a DWI suspect to walk a straight line and touch his nose with his finger is the primary motor cortex.
The motor cortex has to do with movement and balance - so obviously, if you need to prove that you can walk a straight line, which involves both of these things, or if you need to move your finger to a certain spot, which also involves both balance and movement, that would be the correct answer.