Explanation:

- Let's assume the speed of the stream is \( S \).

- The speed of Ravi’s boat in still water is \( 3x \). Therefore, \( S : 3x = 7 : 3 \). Hence, \( S = 7x \).

- The speed of Shyam's boat in still water is \( 9y \). Therefore, \( S : 9y = 5 : 9 \). Hence, \( S = 5y \).

- Equating the speed of the stream from both equations: \( 7x = 5y \).

- Solving for \( x \) and \( y \): \( x/y = 5/7 \).

- The ratio of Ravi’s boat to Shyam’s boat: \( (3x)/(9y) = (3 \times 5)/(9 \times 7)\).

- Simplifying gives: \( 15/63 = 5/21 \).

The correct answer is Option 4: 5 : 21