(A) By adding acid to water with constant stirring.
When acid is added to water slowly with constant stirring, the heat generated is absorbed by the large volume of water, preventing splashing or burns.
The source passage (Chapter 2, Section 2.2.1) explicitly states: "The acid must always be added slowly to water with constant stirring. If water is added to a concentrated acid, the heat generated may cause the mixture to splash out and cause burns." This is a standard safety rule — remember the mnemonic: "Do as you oughta, add acid to water." Option (B) is the dangerous reverse procedure; options (C) and (D) are irrelevant to dilution safety.