Final answer:
Turning to address the client's question concerning the procedure contaminates the sterile field in a nurse's preparation.
Step-by-step explanation:
In the context of a nurse preparing a sterile field, the action that contaminates the sterile field is when the nurse turns to address the client's question concerning the procedure. This is because the nurse's attention is diverted away from maintaining the aseptic technique, which increases the risk of introducing microbes and contaminants to the sterile field.
The other actions mentioned in the options do not contaminate the sterile field. Placing a cotton ball dampened with sterile normal saline on the field does not contaminate the field as long as the cotton ball is sterile. Postponing the procedure for 30 minutes and pouring a liquid into a sterile container from a distance of 25 cm are not actions that directly contaminate the sterile field.