During a rain shower the road surface is likely to be most slippery when

 The road is slippery during the first shower. This is because the road is awash with oils,and other slippery elements. As it showers, these slippery elements do not mix with water,eg oil,but are dispersed over the road making it risky to drive then. As it showers more, however, the slippery elements are washed away, and the roads are safer to drive on.

The correct answer is 3) In the first 30 minutes.

In the first few minutes of the rain, the water on the road is not absorbed and quickly makes the road slippery and unsafe to drive on.

However, as more and more cars keep on driving and as the rain begins to stop...the road will gradually dry up and a lot of the water will be absorbed. 

By the time the shower ends, the roads will not be as slippery as before.

Afine-grained igneous rock forms as a result of slow cooling.