A, according to the image, in the process of A→B, the gas temperature rises and its volume becomes larger.

The volume is proportional to the temperature. From PVT=C, the gas pressure is constant, so A is correct.

B, according to the image, in the process of B→C, the volume remains unchanged and the thermodynamic temperature decreases. According to PVT=C, the pressure P decreases, so B is wrong;

C, as can be seen from the image, in the process of C→D, the gas temperature remains unchanged, and the average kinetic energy of gas molecules remains unchanged, so C is wrong;

D, according to the image, the volume of point D is larger than that of point A, and the total number of gas molecules remains unchanged. The larger the gas volume, the smaller the molecular density, and the molecular density at point D is smaller than that at point A, so D is wrong;

So choose a.