Final answer:
Without a reference to a specific diagram or standard, it is not possible to accurately determine if option a. 4, b. 5, or c. 6 represents a gas solenoid.
Step-by-step explanation:
The symbol that represents a gas solenoid is often specific to the context it is used in, such as electrical engineering schematics or flow diagrams in mechanical systems. However, in the context provided, there's no direct reference to which symbol among the options (a. 4 b. 5 c. 6) specifically represents a gas solenoid. But in general engineering notation, a solenoid valve might be represented by a specific symbol that features a rectangle with two lines indicating the inlets/outlets and possibly another representation for the coil that actuates the valve.
To answer the question accurately, one would usually refer to a standard such as the ISO or ANSI symbols for pneumatic or hydraulic systems if the context is mechanical, or an electrical schematic symbol for solenoids if it's meant for an electrical system. As the provided information does not include these symbols, it's not possible to determine which option (a, b or c) represents a gas solenoid without additional reference material that gives meaning to these numbers.