Answer: B
\(\sqrt{10} - 3 = k\) i.e., \(k = \sqrt{10} - 3\) i.e., \(k(\sqrt{10} + 3) = (\sqrt{10} - 3)(\sqrt{10} + 3)\) i.e., \(k(\sqrt{10} + 3) = (\sqrt{10})^2 - (3)^2\) i.e., \(k(\sqrt{10} + 3) = 10 - 9\) i.e., \(k(\sqrt{10} + 3) = 1\) i.e., \(\sqrt{10} + 3 = \cfrac{1}{k}\)
\(\sqrt{10} - 3 = k\) i.e., \(k = \sqrt{10} - 3\) i.e., \(k(\sqrt{10} + 3) = (\sqrt{10} - 3)(\sqrt{10} + 3)\) i.e., \(k(\sqrt{10} + 3) = (\sqrt{10})^2 - (3)^2\) i.e., \(k(\sqrt{10} + 3) = 10 - 9\) i.e., \(k(\sqrt{10} + 3) = 1\) i.e., \(\sqrt{10} + 3 = \cfrac{1}{k}\)