What would best test the claim that changes in mass are from external atom movement into the organism?

To assess the claim that changes in mass are due to external atom movement into an organism, monitoring the organism's diet for incoming atoms from food and drink is the most effective approach. By analyzing what the organism consumes, one can directly trace the sources of mass gain or change. When an organism ingests food and drink, it takes in various atoms and molecules that contribute to its overall mass. By tracking the types and quantities of substances consumed, it is possible to correlate this intake with changes in mass more accurately.

This method provides clear and tangible evidence supporting the hypothesis that external atoms are increasing the mass of the organism, as opposed to other potential factors influencing mass change. Observing physical activities or measuring temperature, while relevant to other aspects of biology, would not provide direct information about the intake of atoms. Similarly, calculating the volume of the organism's cells does not directly address where the atoms contributing to mass changes originated from. Therefore, investigating the organism's diet stands out as the best approach to establishing a link between external atom movement and mass changes.