A plane electromagnetic wave travelling in vacuum has a magnetic...

Given:
Magnetic field amplitude, $B_0 = 3.0 \times 10^{-8} \text{ T}$
Speed of light, $c = 3 \times 10^8 \text{ m s}^{-1}$

The ratio of the amplitudes of the electric and magnetic fields in vacuum is equal to the speed of light, independent of the wavelength.

Using the relation $c = \dfrac{E_0}{B_0}$, we get:
$E_0 = c \times B_0$
$E_0 = (3 \times 10^8 \text{ m s}^{-1}) \times (3.0 \times 10^{-8} \text{ T})$
$E_0 = 9.0 \text{ V m}^{-1}$

The wavelength data ($600 \text{ nm}$) is extraneous information and is not required for this calculation.

Answer: $9.0 \text{ V m}^{-1}$