The correct answer is A) provide an alternate means of determining the water level, if the gage glass fails. Try-cocks are used on auxiliary boilers to provide a secondary method for checking the water level in the boiler. If the primary water level indicator, the gage glass, were to fail, the try-cocks would allow the operator to visually inspect the water level directly. This is an important safety feature, as maintaining the proper water level is critical for the safe operation of a boiler. The other answer choices are incorrect because they do not accurately describe the primary purpose of try-cocks on an auxiliary boiler. Providing a means for chemical feed, blowing down the gage glass, or acting as a steam sentinel valve are not the main functions of try-cocks.