Final answer:
The correct answer to the proportion of emails opened on smartphones or tablets, according to a US Consumer Device Preference Report, is two-thirds.
Step-by-step explanation:
According to a US Consumer Device Preference Report from Movable Inc., roughly two-thirds of all e-mail is now opened on smartphones or tablets. The options a) one half b) one third c) two-thirds d) three-quarters were provided for selection. The correct answer to this proportion of emails accessed via mobile devices is two-thirds, signifying the shift towards mobile Internet usage and the importance of optimizing emails for mobile viewing.