The equation $y=a{\cos }^2(2\pi nt-\frac{2\pi x}{\lambda })$  represents a wave with  

amplitude a/2, frequency 2n and wavelength $\lambda$

amplitude a, frequency n and wavelength  $\lambda$

amplitude a, frequency 2n and wavelength $2\lambda$ 

amplitude a/2, frequency 2n and wavelength  $\lambda /2$