In this question in first line it is written that a,b,c,d,e,f are live at entry point. Which is wrong, as e is not live at entry point.
Variables live at exit point ={}=phi. ( Because this is the end of program , all variable will be dead here.)
Variables live at path 1 of entry point = {a,b}
Variables live at path 2 of entry point = {c,d,f}
Variables live at path 3 of entry point = {b,c}
Variables live at entry point = Variables live at path 1 of entry point Union Variables live at path 2 of entry point Union Variables live at path 3 of entry point. = {a,b} U {c,d,f} U {b,c} = {a,b,c,d,f}
May be they want to ask the number of variable live at entry point. Anyhow you are right.