Step 1: Understanding the Concept
An echo returns after the sound travels to the cliff and back, so the distance to a cliff is \(\frac{vt}{2}\).
Step 2: The two cliffs
The first echo gives \(d_{1}=\frac{3v}{2}\) and the second gives \(d_{2}=\frac{5v}{2}\).
Step 3: Calculation
The cliffs are \(1360\) m apart, so \(d_{1}+d_{2}=\frac{3v+5v}{2}=4v=1360\).
Step 4: Result
\(v=340\) m/s, which is option (C).
Final Answer:
The speed of sound is 340 m/s.
\[ \boxed{\text{(C) }340\text{ m/s}} \]