Relative clauses put in who, that or which where necessary where the relative pronoun can be left out put a cross (x)
1- you are the only person can help us.

The correct answer is option a.

is basically a good source of getting and others by what you know for example if someone asked a question and you know the answer you could answer it and get some points or if you need on a question you could make a post about it and wait till someone answers it or just look up the question and see if someone else has asked about it