Answer :
A. On the other hand
The phrase which signals a contrast between ideas about marriage is "on the other hand" because the part before was about the American consideration and ideas about love, while the part following "on the other hand" explain why in other cultures the idea of romantic love is not at the base of marriage.
The phrase which signals a contrast between ideas about marriage is "on the other hand" because the part before was about the American consideration and ideas about love, while the part following "on the other hand" explain why in other cultures the idea of romantic love is not at the base of marriage.