What is the most efficient action to take if you are away from your desk and expecting an important email from a guest speaker?

Downloading and/or activating the Gmail mobile app is the most efficient action to take when you are away from your desk and expecting an important email. This answer is correct because having the mobile app installed on your device allows you to receive real-time notifications for incoming emails, ensuring you don't miss the important message from the guest speaker. The app provides full access to your email account, enabling you to read and respond to emails directly from your mobile device, regardless of your location.

Checking email on a colleague's computer may not be an efficient approach due to potential delays in getting access or having to wait for your colleague to be available. Additionally, it might lead to privacy concerns, as you would be using someone else's device to access personal email.

Sending a reminder email to the speaker is less efficient in this scenario because it doesn't guarantee that you will receive the important email you are awaiting. While following up can be beneficial, it diverts attention from checking for the email you are expecting.

Using a student’s device to check your email can also be problematic. Similar to using a colleague's computer, it raises privacy issues and may not provide a reliable or secure way to access your sensitive emails. Having your personal device with the Gmail app is more convenient and secure.