Which of these requirements is non$-$functional?

Question $6$Select one:

a$.$

A program should allow a user to choose a color of interface.

b$.$

A program should request a user to update profile data, i$.$e$.,$ name, e$-$mail, country, photo$/$avatar$,$ every $6$ months.

c$.$

A program should allow a user to upload and install additional fonts from official website.

d$.$

A program should support $10$ color schemes for a graphical user interface.