Traveling Salesman Problem: Branch & Bound $($Exit Vertices First$)$

Given an instance of the TSP problem $($Shortest Distance$),$ which of the following paths represents the right way to

compute the upper bound of $ACF$ :

Select one:

a$.B\{A,C,D,G\},F\{F,C,D,G\},C\{B,A,D,F\},D\{B,A,C,G\},G\{B,A,C,D\}$

b$.F\{B,C,D,G\},B\{A,C,D,G\},C\{B,A,D,G\},D\{B,A,C,G\},G\{B,A,C,D\}$

c$.F\{B,E,D,G\},B\{A,E,D,G\},D\{B,A,E,G\},E\{B,A,D,G\},G\{B,A,E,D\}$

d$.B\{F,C,D,G\},C\{F,B,D,G\},D\{B,C.G,F\},G\{B,D,C,F\},A\{B,G,C,D\}$