Final answer:
In an engineering vacuum system, the requirement for each station inlet is a primary valve, which regulates the vacuum pressure and flow to the station.
Step-by-step explanation:
The question: What is a requirement for each station inlet? refers to a component within a vacuum system or something similar in engineering. When discussing a station inlet for such systems, the requirement is usually a method to control the vacuum pressure and flow to the specific station. The correct answer is C) a primary valve. This valve's function is to regulate the intake of the vacuum at the station, ensuring that the correct amount of vacuum pressure is available for the particular application. It is typically found at the point where the station inlet connects to the main vacuum line and acts as the immediate control for that station. Other options, such as a primary optional in line (B) or a secondary demand valve (D), may exist within the system but do not serve as the required component for the inlet itself.