A siren on an ambulance emits sound at a constant frequency of 750 Hz. The ambulance is travelling at a constant speed of 25-0 ms⁻¹ towards a stationary observer. The speed of sound in air is 340 ms⁻¹. The frequency of the sound heard by the observer is

The frequency of the sound emitted by the siren (source frequency) is $f_s = 750 \text{ Hz}$.
The speed of the ambulance (speed of source) is $v_s = 25.0 \text{ m/s}$.
The speed of sound in air is $v = 340 \text{ m/s}$.
The observer is stationary, so the speed of the observer is $v_o = 0 \text{ m/s}$.
The ambulance is moving towards the stationary observer.

For a source moving towards a stationary observer, the observed frequency $f_o$ is given by the formula:

First, calculate the denominator:

Now, substitute this back into the equation:

The calculated frequency is approximately $809.52 \text{ Hz}$. Looking at the options:
A: 695 Hz
B: 699 Hz
C: 750 Hz
D: 805 Hz
E: 810 Hz

The closest option to $809.52 \text{ Hz}$ is $810 \text{ Hz}$.

The final answer is $\text{E}$.