Answer: The variables which remain constant while testing Boyle's law is temperature and number of moles.
Step-by-step explanation:
Boyle's law states that pressure of the gas is inversely proportional to the volume of the gas at constant temperature and number of moles.
Mathematically,
(At constant temperature and number of moles)
or,

With increase in pressure, the volume of the gas decreases and vice-versa.
Hence, the variables which remain constant while testing Boyle's law is temperature and number of moles.