**Explanation:**

When an ideal gas is heated at constant pressure, the volume of the gas increases. This can be explained by the ideal gas law, which states that at constant pressure, the volume of a gas is directly proportional to its temperature.

According to the ideal gas law, PV = nRT, where P is the pressure, V is the volume, n is the number of moles of gas, R is the ideal gas constant, and T is the temperature in Kelvin.

When the gas is heated at constant pressure, the pressure remains constant, so we can rewrite the ideal gas law as V/T = nR/P. Since the number of moles of gas (n) and the ideal gas constant (R) are constant, the equation simplifies to V/T = constant.

From this equation, we can see that as the temperature increases (T), the volume (V) must also increase in order to keep the ratio V/T constant. This means that when an ideal gas is heated at constant pressure, the volume of the gas increases.

The other options provided in the question are not correct:

- The mass of the gas remains the same because heating a gas does not change the number of gas particles or their mass.

- The kinetic energy of the molecules increases as the temperature increases, but this does not necessarily mean that it increases when the gas is heated at constant pressure. The kinetic energy of the gas molecules can increase or decrease depending on the specific conditions.

- The attraction forces between gas particles do not increase when the gas is heated at constant pressure. In fact, heating a gas often weakens the intermolecular forces between the gas particles, leading to increased molecular motion and increased volume.

Therefore, the correct answer is option 'A': The volume increases when an ideal gas is heated at constant pressure.