To find the speed of sound in air, we can use the information given about the distance to the cliff and the time it takes for the echo to be heard.
- The distance to the cliff is 633 meters.
- The hiker shouts, and the sound travels to the cliff and back. Therefore, the total distance traveled by the sound is \(2 \times 633 , \text{m} = 1266 , \text{m}\).
- The time taken for the echo to return is 4.0 seconds.
Now we can use the formula for speed, which is:
\[ \text{Speed} = \frac{\text{Distance}}{\text{Time}} \]
Plugging in our values:
\[ \text{Speed} = \frac{1266 , \text{m}}{4.0 , \text{s}} = 316.5 , \text{m/s} \]
Rounding to the nearest whole number, the speed of sound in air is approximately 317 m/s.
Thus, the correct choice is:
317 m/s.