The best practice that Karan should adopt is:
D.
disclose to the employees that their emails will be monitored
Step-by-step explanation:
To monitor the social network activities of the workers of a company is walking the line of ethics so there is a way to determine what must be the best practice keeping in line with the working culture of the company.
The people working there must be informed that their activity will be monitored so they have an understanding that it will be done.
After this one must perform checks either arbitrarily or on the basis of who seems more trustworthy and who less.