Answer:
Favorite sport .
Step-by-step explanation:
A school mascot would be unsafe to be associated with your online username as potential hackers and strangers would have an idea where school do you attend.
Year of birth would also be unsafe as it would give them an idea when you were born. Also, birthdates are often part of registrations in websites and even in the offline world.
City of residence is also unsafe to be used as you would give attackers and online strangers to know where do you live.