To me, the key to this task lies within understanding its focal message. What did the author intend to convey?
It seems that the message goes as follows: The republic was progressive, because it granted full voting rights when it was founded - and it happened in 1755.
Which answer options illustrate exactly that?
Both A and C focus on the date more than the message, so I eliminate them.
Then, D sends the right message, but the preposition is wrong - it doesn't sound right, when you are progressive 'by doing sth'. So, we eliminate D as well.
We have a choice between B and E then - and although we can sometimes express a reason with 'for', in this case 'in deciding' sound best stylistically. It's just a very idiomatic figure of speech: 'to be <adj> IN doing sth', which is used exactly to highlight how an action brings out a certain characteristic in the actor.
So, idiomatically, the right answer is E.