Traiga el pan Ud.
Why?
B) Trae could be correct, but it's present tense, not future tense.
C) Traen is past present tense, not future tense.
D) Traigan is for a group, here it's only one person, Ud.
This is how we get A) Traiga, because we use the present tense and the one-person word.
Hope this helps!~