Final answer:
An informal note is used in situations when a co-worker is busy, away from their desk, or the message needs immediate attention, which makes 'd. all of the above' the correct option.
Step-by-step explanation:
An informal note is used when d. all of the above situations occur. Such notes can be used when a co-worker is busy and you need to leave a quick message without disrupting their workflow, when they are temporarily away from their desk and you want to leave a message for their return, or when a message should get immediate attention, but the situation doesn't warrant a formal communication or the formality of an email.
Informal notes are part of office etiquette, which encourages open and friendly communication while maintaining professionalism. When writing an informal note, it is essential to be clear and concise, and ensure that the note is left in a place where the co-worker can easily find it when they return to their workspace.
Informal notes are often used for quick updates, reminders, or to convey information that does not need to be part of the official record.