The terms simplify: $\sqrt{18}=3\sqrt{2}$, $\sqrt{50}=5\sqrt{2}$, $\sqrt{98}=7\sqrt{2}$, so $d=2\sqrt{2}$. The 4th term $=7\sqrt{2}+2\sqrt{2}=9\sqrt{2}=\sqrt{162}$. Answer: (C) $\sqrt{162}$
Rewrite each term as a multiple of $\sqrt{2}$: the AP is $3\sqrt{2}, 5\sqrt{2}, 7\sqrt{2}, \ldots$ with common difference $2\sqrt{2}$. The 4th term is $9\sqrt{2} = \sqrt{81 \times 2} = \sqrt{162}$. Note the options are under the square root — don't forget to square back when matching the answer.