Statement I states that if temperature rises, the coefficient of viscosity of a liquid decreases. Viscosity is a property of fluids that describes the internal friction or resistance to flow. In most fluids, including liquids, as the temperature increases, the viscosity decreases. This is because an increase in temperature causes the molecules within the liquid to move faster, reducing the interactions between them and thus decreasing the resistance to flow. Therefore, statement I is correct.

Statement II states that the velocity of a falling raindrop attains a constant value because of the viscous force exerted by air. When a raindrop falls through the air, it experiences two main forces - gravity pulling it downwards and air resistance acting in the opposite direction. The viscous force due to air resistance tends to oppose the motion of the raindrop and eventually balances out the gravitational force, leading to a constant velocity called the terminal velocity. Therefore, statement II is also correct.

Hence, both statement I and statement II are correct. Therefore, the correct answer is option 3.