What key must be selected before adding or inserting lines of code?

Selecting the INSERT key is essential before adding or inserting lines of code because this key typically toggles the text input mode in many programming environments or text editors. When the INSERT mode is activated, it allows users to insert new characters or lines without overwriting existing ones. This is crucial in programming and code editing, where maintaining the integrity of the existing code while adding new functionality is often necessary.

In contrast, other options do not serve this particular purpose. The ENTER key, for example, is primarily used to execute commands or finalize input rather than toggle the input mode. The ASSIST key is not a standard key associated with text editing and does not provide a mechanism for adding lines of code. The MODIFY key may imply editing capabilities, but it does not specifically denote the action of inserting new lines. Thus, using the INSERT key is the correct and purposeful choice for this action in a coding environment.