Why B? any explanation?

Historians believed that by implementing contract would bring gain's for women, however these gains didn't elevate their status as they were still casually treated in society. Therefore the premise that they believed in initially didn't work out the way they expected it to, a state of paradox eventuality.

neat explanation

The first sentence:

In the west in the late seventeenth and eighteenth centuries, marriage began to incorporate certain features of a contract
- It presents a FACT which two distinct segments: an adverbial phrase of time (in the....centuries) AND a complete caluse (marriage....contract).
- From this, the highlight part (marriage began to incorporate certain features of a contract) plays a role of an objective premise. NO subjective evaluation (cause or effect), opinion (wish or belief), suggestion (attempt or implication), etc. of any kind is presented.

Only choice B and C are qualified.

The second sentence:

Historian have traditionally argued that this trend represented a gain for women; but whatever gains marriage contracts may have meant for women were undermined by social nonchalance

- Using grammatical structure alone, this sentence consists of two CONTRASTING clauses with the first clause (historian....women) presents an argument (or a belief) of historian depriving from the first sentence (the consequence of incorporating features of a contract to marrigae) AND the second clause ( but whatever gains marriage contracts may have meant for women were undermined by social nonchalance ) clearly contradicts any thinking that such gains from that incorporation would really benefit women because whatever the gains were, the were undermined by social nonchalance.

Only choice B is perfectly fit.

I hope this helps.