Opening a dropdown presents a simple list of textual options. These may be divided into logically related groups, and a single option may be indicated as selected with a checkmark.
Logically related options may be grouped together for easy scanning.
When a dropdown contains actions that edit some piece of data, you may use a checkmark to indicate the current state of the value being edited.
In most cases you should avoid putting an excessive number of items in the dropdown menu. Occasionally the use case will require it (for example, a list of times in a time picker). If necessary, the length of the menu may be capped and a scroll bar will then allow access to all items.