Which operation correctly describes assembling the trigger housing assembly?

The operation that effectively describes assembling the trigger housing assembly involves the correct sequence of inserting components to ensure proper functionality. Choosing the option that includes "Insert the holding notch, rotate upwards, and insert the spring pin" accurately reflects the necessary steps required for assembly.

In this scenario, the holding notch is a crucial component that must be positioned correctly to allow for the upward rotation, which facilitates the incorporation of the spring pin into the assembly. This sequence ensures that all parts are aligned properly, which is essential for the mechanism to function effectively. Proper rotation is required to engage the components securely, thus ensuring operational integrity.

The other choices, while containing some relevant steps, do not capture the correct sequence or critical components necessary for assembling the trigger housing assembly effectively. For instance, options that mention attaching screws or pushing down may involve ancillary steps but fail to outline the essential interactions that must occur first, such as inserting the holding notch and securing it with the spring pin. Therefore, the correct answer emphasizes the proper method required for a successful assembly.