To allow the scientist to change the rate of the reaction so the results could be measured in small increments, the most suitable option would be an inhibitor.
An inhibitor decreases the rate of a reaction, allowing for the release of gas to be measured more gradually. On the other hand, an enzyme or an accelerator would increase the reaction rate, making it more challenging to measure the gas release in controlled increments. A depressant typically refers to substances that reduce physiological or nervous activity and is not typically applicable in this context of chemical reactions.