Determine the wavelength (lambda) from the diagram.

The diagram shows 3 complete cycles of the wave over a distance of $3 \, \text{m}$.
Therefore, the length of one wavelength is:

$$\lambda = \frac{3 \, \text{m}}{3} = 1 \, \text{m}$$

The wave travels with a speed of $300 \, \text{ms}^{-1}$.

$$v = 300 \, \text{ms}^{-1}$$

The relationship between wave speed, frequency, and wavelength is given by:

$$v = f \lambda$$

To find the frequency, rearrange the formula:

$$f = \frac{v}{\lambda}$$

Substitute the values:

$$f = \frac{300 \, \text{ms}^{-1}}{1 \, \text{m}}$$ $$f = 300 \, \text{Hz}$$

The calculated frequency is $300 \, \text{Hz}$. This matches option B.

The final answer is $\text{B. 300Hz}$.