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In an indirect air-conditioning system, hot water is utilized to heat the air. This method typically involves a heat exchanger, where the water is heated by a boiler or other heating source and then circulates through the heat exchanger. The heat from the water is transferred to the air without the two coming into direct contact, hence the term "indirect." This system is efficient in maintaining a consistent temperature while allowing for effective distribution of warmth throughout a space.

The other options do not fit well within the context of how indirect air-conditioning systems function. Electric coils are often associated with direct heating systems, while hot air and steam may be used in other types of heating systems or in direct heating applications rather than in an indirect capacity. Thus, hot water stands out as the most appropriate choice for heating air in an indirect air-conditioning system.