Final Answer:
Primary engineering control pre-filters must be changed every c. 30 days.
Step-by-step explanation:
In environments where primary engineering controls, such as cleanrooms or biosafety cabinets, are utilized, the pre-filters play a crucial role in maintaining air quality. The recommended frequency for changing primary engineering control pre-filters is typically every 30 days (Option C).
This regular replacement helps ensure the effectiveness of the filtration system in capturing particulate matter and contaminants, preventing their release into the controlled environment. Options A, B, and D do not align with the standard practice for pre-filter replacement intervals and may compromise the cleanliness and integrity of the controlled environment if not adhered to.
Option C is the answer.