The correct answer is option 3 - the product of the pressure and volume of an ideal gas is directly proportional to its temperature.

To understand this concept, we need to look at the equation of state for an ideal gas, which is the relationship between its pressure, volume, and temperature. The equation is known as the ideal gas law and is given by PV = nRT, where P is the pressure, V is the volume, n is the number of moles of gas, R is the universal gas constant, and T is the temperature.

In this equation, we can see that the pressure and volume are multiplied together on one side of the equation. This means that any change in pressure or volume will also affect the product of the two.

On the other hand, temperature is on the other side of the equation, and if we look at the equation as a whole, we can see that temperature is directly proportional to the product of pressure and volume. This means that if we increase the temperature of an ideal gas, the product of its pressure and volume will also increase, and vice versa.

So, the correct answer is option 3 because the product of the pressure and volume of an ideal gas is directly proportional to its temperature.