Final answer:
E-mails should have a polite and courteous tone, being friendly and respectful without being too familiar. The tone of most e-mails is semi-formal.
Step-by-step explanation:
The tone of most e-mails should be polite and courteous. When communicating via email, it's important to be friendly and respectful, while maintaining a level of formality that respects the boundaries of a professional or semi-formal relationship. It's recommended to avoid humor or sarcasm, as these can often be misunderstood in written form, lacking the context and tone of voice present in face-to-face communication. While a more casual tone might be acceptable in certain contexts, such as an email to a friend, when in doubt, erring on the side of formality can help make a good first impression and avoid miscommunication.
The correct answer to the question - The tone of most e-mails is semi-formal (d), which strikes a balance between being overly formal and too casual, making it suitable for a wide range of email communications.