What type of HVAC system uses ducts to distribute air?

The correct choice is a ducted system. This type of HVAC system specifically utilizes a network of ducts to deliver conditioned air throughout a building. Ducts are crucial in a ducted system as they serve the purpose of directing the airflow from the HVAC unit to various spaces within the structure, ensuring efficient heating and cooling.

In a ducted system, conditioned air produced by a central unit, such as a furnace or air conditioner, travels through the ductwork to reach different rooms, allowing for even distribution of temperature. This method is particularly effective in larger buildings where multiple spaces need to be serviced.

Each of the other options represents different HVAC technologies or methods that do not primarily rely on ductwork for air distribution. For instance, a hydronic system uses water to transfer heat and does not involve air ducts; a split system consists of components that handle air conditioning and heating in separate units but can be either ducted or ductless; and portable systems are standalone units that do not use ductwork, providing localized conditioning in a single space. Thus, the defining characteristic of a ducted system is its reliance on ducting to distribute air, distinguishing it from the other systems listed.