Answer
The de Broglie wavelength is increasing.
Work Step by Step
We can use Equation (38-20) to find an expression for the de Broglie wavelength:
$\frac{1}{2}mv^2 = \frac{h^2}{2m \lambda^2}$
$\lambda^2 = \frac{h^2}{m^2~v^2}$
$\lambda = \frac{h}{m~v}$
An electron has a negative charge. Since the electron is moving in the same direction as the electric field, the speed is decreasing.
Therefore, the de Broglie wavelength is increasing.
