Question

In which situation is the acceleration of the car negative?

Answer 1

The car's acceleration is negative if ...

-- it's going slower and slower in the direction that you call 'positive',

OR . . .

-- it's going faster and faster in the direction opposite
to what you call 'positive',

OR . . .

-- it's driving around in a circle AND you call the direction
OUT from the center of the circle the 'positive' direction. 

Answer 2

When a car is slowing down, it has a negative acceleration.  Although it is not going a negative speed, it is decreasing in velocity, which is the definition of a negative acceleration.

Hope this helps!