In 1860, the Philological Society launched its effort to create a dictionary more comprehensive than the world had ever seen; although the project would take more than 60 years to complete, the Oxford English Dictionary had been born.
a) would take more than 60 years to complete, the Oxford English Dictionary had been
b) took more than 60 years to complete, the Oxford English Dictionary was
c) would take more than 60 years to complete, the Oxford English Dictionary was being
d) would take more than 60 years to complete, the Oxford English Dictionary was
e) took more than 60 years to complete, the Oxford English Dictionary was about to be
IMO the answer is D
Here are my reasons -
Here I am logically assuming that the birth of the dictionary signifies the launch of the project and this took place before the the entire project was completed. Since these events are taking place at different times, we need to have perfect past tense rather than simple past. Therefore ELIMINATE B
Eliminate C ('Being)
Eliminate E (about to be completed is incorrect as event has finished in the past)
Eliminate A (wrong usage of tenses)
Now look at D
D correctly uses "would take" to signify that the completion event took place after the birth. Although "had been" here would have sounded better (for perfect tenses), "would take" is a classical case of 'future in past tense' (according to the frame of time reference identified by the non-underlined portion) hence we have to use 'would' rather than 'will'. Normally we cannot use 'would' to express facts but here the events have completed in the past.
Hope this helps. Consider Kudos for the post