Answer:
C. keep the temperature constant so the collected gas will not expand
Step-by-step explanation:
A water bath is an instrument in the laboratory used for controlled temperature experiment. It is used to maintain a constant temperature of volume of samples of water, air or chemicals. It consist of water being heated at a regulated and required temperature. Where the volume of the sample is placed in the water bath during an experiment so as to avoid unnecessary expansion of the sample.
In the question given, the purpose of putting the flask in a water bath during the experiment is to ensure that the content of the flask is at a constant temperature and avoid unnecessary expansion. Thus the correct option is C.