Hot and cold aisle containment prevents mixing of supply and return air, which improves cooling efficiency.
By maintaining higher return-air temperatures, cooling units operate more efficiently, often allowing higher chilled water setpoints. This reduces overall cooling power consumption, directly improving PUE (Power Usage Effectiveness).
Containment is recognized by ASHRAE and Green Grid as one of the simplest and most cost-effective methods for lowering PUE. Options A, C, and D are false because containment benefits apply regardless of cooling source or IT virtualization.
References: ASHRAE TC 9.9 - Airflow Management, The Green Grid White Paper #42.