This question tests relevance: before combining any statements, check if they actually talk about the person or quantity the question asks about. Here the question wants Mrs. X's tax amount.
Now combine both statements: together they only build a fuller picture of Gauri and Rajat's tax payments over two years. Since nothing in either statement, or in the two put together, links Gauri or Rajat's numbers to Mrs. X, there is no way to work out what Mrs. X paid.
Let's summarize:
So even both statements taken together fail to answer the question, which makes option (4) correct.