The de Broglie wavelength of a moving particle is inversely proportional to its:

The de Broglie wavelength ($\lambda$) of a moving particle is given by the formula:

$$\lambda = \frac{h}{p}$$

where $h$ is Planck's constant and $p$ is the momentum of the particle.

From the formula, it is clear that the de Broglie wavelength ($\lambda$) is inversely proportional to the momentum ($p$). As momentum increases, the wavelength decreases, and vice versa, assuming Planck's constant is a fixed value.

The final answer is $\text{B. Momentum}$.