b

$$\begin{array}{r}{b}_n:=\frac{(\cos (n){)}^2}{\sqrt{n}}\\ \underset{n\to \infty }{\lim }{b}_n=\text{?}\\ \to \text{Sandwichtheorem}\\ a_n:=0\\ \underset{n\to \infty }{\lim }{a}_n=0\\ c_n:=\frac{1}{\sqrt{n}}\\ \underset{n\to \infty }{\lim }{c}_n=0\\ \stackrel{(3)}{⟹}\underset{n\to \infty }{\lim }{b}_n=0\end{array}$$