B, if fat is used as the respiratory substrate, and the beaker of device A is filled with clean water, it will lead to the reduction of gas volume, and the emission of CO2 cannot be measured. B is wrong;
C, a beaker filled with water, because the volume of gas may become larger, unchanged or decreased, the gas volume change is measured. If the device B is replaced with a CO2 absorbent, the O2 consumption is measured, and the amount of released carbon dioxide is equal to the gas volume change+oxygen consumption, and C is correct;
D, measuring the consumption of oxygen can only use carbon dioxide absorbent to absorb carbon dioxide, and then measuring the volume reduction is the absorption of oxygen, D is wrong.
So choose: C.