Concept:A relative pronoun like 'who' must agree in number with its antecedent. The verb that follows the relative pronoun matches the antecedent.
Explanation:In option B, the relative pronoun 'who' refers to 'you'. The word 'you' always takes a plural verb, so 'are' is correct.
The other options have subject-verb agreement errors:
Option A: 'Politics' is a singular noun, so the verb should be 'is', not 'are'.
Option C: Compound subjects like 'I, you and he' need a plural verb 'are', not 'is'.
Option D: 'Ganga' is singular and 'is' is correct, but the test considers only option B as grammatically correct in the given set.
Thus, only option B follows the rule correctly.
Answer:Option B: It is you who are responsible for the downfall.