By the time we have reached France, we will have been backpacking for 12 weeks
a. same
b. by the time we have reached France, we will have backpacked for 12 weeks.
c. by the time we reach France, we will have been backpacking for 12 weeks.
d. by the time we will have reached France, we will have backpacked for 12 weeks.
e. by the time we reached France, we will have been backpacking for 12 weeks.
Tense error
I would vote for D. Both are in Future Perfect tense. B doesnt make complete sense.