The correct answer is B. to.
The word “indifferent” means “not caring about or interested in something.” In the context of the sentence, “One should not be indifferent to the sufferings of others,” the speaker is saying that we should not ignore or be unconcerned about the pain and hardship that others are going through.
The other options are not as appropriate in this context. Option A, “about,” could be used to describe someone who is not very knowledgeable about something. Option C, “of,” could be used to describe someone who is not a fan of something. Option D, “at,” could be used to describe someone who is angry or annoyed at something.
It is important to be aware of the suffering of others and to do what we can to help them. We can do this by donating to charities, volunteering our time, or simply being there for someone who is going through a difficult time.