The correct option is B At
Here, 'at' is correct because it implies in the direction of or towards somebody/something. Here, it is the physically challenged.
'On' is incorrect because it implies in or into a position covering, touching or forming part of a surface.
'Of' is incorrect because it implies coming from a particular background or living in a place.
'In' is incorrect because it implies at a point within an area or a space.