If you don’t immediately see what’s going on, always try calculating $y_n$ for small $n$:
$\begin{align*} y_1&=\sqrt 1=1\\ y_2&=0\\ y_3&=\frac{\sqrt2}3\\ y_4&=0\\ y_5&=\frac{\sqrt3}5\\ y_6&=0\\ y_7&=\frac{\sqrt4}7 \end{align*}$
Look for patterns, and then try to see why they’re there. Here you shouldn’t have too much trouble spotting the patterns and writing down a general formula for $y_n$ when $n$ is even and another for $y_n$ when $n$ is odd. Even with the seven values that I’ve calculated here you should be able to explain why (a) is not the answer, and once you figure out the general formulas, the right answer is easy to pick out.