IMO "B" is better answer.

I picked B too, but it's wrong I don't understand why!

Hmmm

First some revision of concepts is needed here-
When/Where do we use past perfect(or other complex tenses)?

The past perfect (or other complex tenses) is used when there are two past actions and we want to indicate which one happened first(or some chronological list/events)

Read the first part(one preceding the semicolon) of the sentence again.
In 1860, the Philological Society launched its effort to create a dictionary more comprehensive than the world had ever seen;

you have to bear in mind that the time frame here is 1860. Some decision has been taken, some start has been made that would lead to some future event/product.[Now read on second part as if the time frame is still 1860]. The completion of project is still a futuristic event(the day when the dictionary would actually be completed) but the 'start' of the project has happened already("born" signifies the start of the project here).

Hence we write
although the project would take more than 60 years to complete, the Oxford English Dictionary was

D it is

Cheers!

Thank you reachac! Your explanation is awesome!
The use of "would" form confused me the most, but now I got it!