Which type of hydrant opens a small drain valve as it is closed?

The correct choice is a dry-barrel hydrant. A dry-barrel hydrant is designed primarily for areas where freezing temperatures occur. Its unique construction includes a small drain valve that automatically opens when the hydrant is closed. This feature allows any residual water left in the hydrant to drain out, preventing freezing and potential damage during cold weather.

In contrast, wet-barrel hydrants are used in warmer climates and do not have a drain valve; water remains in the barrel of the hydrant at all times. Combination hydrants typically include features of both wet and dry-barrel designs but are more versatile than specifically focused on preventing freezing. Standpipes, on the other hand, are part of a fire protection system that provides a means for firefighters to access water but do not utilize a hydrant mechanism. The design and functionality of a dry-barrel hydrant make it specifically suited for cold climates where freezing would be a concern, thereby making it the correct answer to this question.