The correct option is
A On
Option A: 'On' refers to the 'top or surface of something, or being connected to it'. It relates to the given sentence: The vehicles are on (connected to the surface of) the roads. Hence option A is correct.
Option B: 'From' indicates 'a source or origin', or 'a starting point'. The sentence does not indicate any source.
Option C: 'At' indicates a point or place occupied in space.
When referring to an address, we usually use 'at' for the road. When referring to a particular part of the road, we use 'at': 'at the side of the road', 'at the end of the road'.
It doesn't apply here since the noun 'vehicle' is spoken of as actually being ON the road.
Option D: 'For' means 'with the object or purpose of'. The sentence does not express purpose.
These words do not complete the sentence in meaning. Hence options B, C and D are incorrect.