TOPIC: "bit"

When I am going to taking image, someone said to me below:
A. Sorry, but you look a little too serious! Could you possibly smile?
B. Sorry, you look a little too serious!
C. Sorry, you look a little serious!
D. Sorry, you look a bit serious!
I am not easy to accept using " bit", could you help me, it is strange for me.

About your querie "Look serious" and whether there are differences in these sentences:
B) Sorry, you look a little too serious!
C) Sorry, you look a little serious!
D) Sorry, you look a bit serious!
Yes, these are all good examples and they have about the same meaning, although B is a bit more negative. Don't be afraid of using " a bit" to mean "a little", as it is very, very common, especially in spoken English.