Between 4 º C to 0 º C water behave in an abnormal manner and expands. Expansion leads to an increase in volume, and when volume increases considering the mass is constant density decreases. Density is inversely proportional to volume(density=mass/volume) .So the assertion is true. When the volume of matter for the same mass increases its molecules should be spread apart even when in solid-state, which is opposite to what has been stated in the reason. Therefore, the reason is false. Hence, "A is true but R is false." is the correct answer.