The correct option is B To
Here, 'to' is correct because it implies before the start of something, here it is the marriage of ....
'On' is incorrect because it implies in or into a position covering, touching or forming part of a surface.
'For' is incorrect because it is used to show who is intended to have or use something or where something is intended to be put.
'At' is incorrect because it is used to say where something/somebody is or where something happens.