Question

respirometers measure
the rate of breathing through the lungs.
the rate of photosynthesis in lentil seeds that are germinating.
the rate of cellular respiration in an organism.

Explanation:

Respirometers are used to measure the rate of cellular respiration in an organism. They work by measuring the exchange of oxygen and carbon - dioxide which are key components of cellular respiration. They do not measure the rate of breathing through the lungs (which is a different physiological process at the organ - level), nor the rate of photosynthesis (which occurs in plants and is a completely different metabolic process from respiration).

Answer:

The rate of cellular respiration in an organism