The spatial variation of the electric field of a plane...

From the given description of the spatial variation, the peak value of the electric field is its amplitude, $E_0 = 60 \text{ V m}^{-1}$.

The spatial period (wavelength) is extra information and is not required to find the magnetic field amplitude.

Using the relation between the amplitudes of the electric and magnetic fields in vacuum:

$B_0 = \dfrac{E_0}{c}$

$B_0 = \dfrac{60}{3 \times 10^8} = 20 \times 10^{-8} \text{ T} = 2.0 \times 10^{-7} \text{ T}$

Answer: $2.0 \times 10^{-7} \text{ T}$