A good reason to contact an instructor by email would be to ______.

Contacting an instructor by email is appropriate for a variety of academic-related reasons, one of which is to turn in an assignment that can be submitted through email. Email serves as a direct and formal channel for submitting assignments, particularly when the online course platform might have technical issues or when students are required to submit a document in a specific format. This method allows for clear documentation of the submission, ensuring both the student and instructor have a record.

In the context of the other options, while discussing course content, asking about grades, or setting up a meeting can also be valid reasons to reach out, they do not specifically highlight the formal nature of submitting work through email in the same direct way that option B does. Each of these situations involves different protocols that may or may not require email contact, such as using course forums or scheduling tools. However, the act of submitting assignments is more straightforwardly tied to the use of email as a formal means of communication within this academic context.