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This question involves the Doppler effect, where the observed frequency changes depending on the relative motion between the source (train) and the observer.

### Doppler Effect Formula:
When the source (train) moves towards the observer, the observed frequency \( f_1 \) is given by:
\[
f_1 = f_0 \left( \frac{v + v_o}{v - v_s} \right)
\]
Where:
- \( f_0 \) is the emitted frequency of the whistle,
- \( v \) is the speed of sound in air (340 m/s),
- \( v_o \) is the speed of the observer (0 m/s, since the observer is stationary),
- \( v_s \) is the speed of the source (train).

For a stationary observer, this simplifies to:
\[
f_1 = f_0 \left( \frac{v}{v - v_s} \right)
\]

Now, the frequency observed at two different speeds:
1. When the train moves at 34 m/s, the observed frequency is \( f_1 \):
   \[
   f_1 = f_0 \left( \frac{v}{v - 34} \right)
   \]
2. When the train's speed reduces to 17 m/s, the observed frequency becomes \( f_2 \):
   \[
   f_2 = f_0 \left( \frac{v}{v - 17} \right)
   \]

We are asked to find the ratio \( \frac{f_1}{f_2} \):
\[
\frac{f_1}{f_2} = \frac{ \frac{v}{v - 34} }{ \frac{v}{v - 17} } = \frac{v - 17}{v - 34}
\]
Substituting \( v = 340 \, \text{m/s} \):
\[
\frac{f_1}{f_2} = \frac{340 - 17}{340 - 34} = \frac{323}{306} = \frac{19}{18}
\]

Thus, the correct ratio \( \frac{f_1}{f_2} \) is **19/18**, and the correct option is (D).

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Would you like further details on this? Here are some related questions:

1. How does the Doppler effect apply to other types of waves (like light)?
2. What happens to the observed frequency if the train moves away from the observer?
3. How would the result change if the observer was moving as well?
4. How does the speed of sound affect the frequency change in the Doppler effect?
5. What would the frequency ratio be if the train's speed were doubled instead?

**Tip:** In Doppler effect problems, it's essential to differentiate between the source's and observer's movement and how they influence the frequency heard by the observer.

A train moves towards a stationary observer with speed 34 m/s. The train sounds a whistle and its frequency registered by the observer is f_1. If the train’s speed is reduced to 17 m/s, the frequency registered is f_2. If the speed of sound is 340 m/s, find the ratio f_1/f_2.

Learn how to solve a Doppler effect problem involving a train moving towards a stationary observer. The problem focuses on calculating the frequency ratio based on the train's speed and the speed of sound. Suitable for high school physics students.

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