Step 1: Understanding the relationship.
Let Rohit's current age be \( 5x \) and Vivek's current age be \( 3x \), as the ratio between their ages is 5:3. We are also told that Vivek is 8 years younger than Rohit, so:
\[
5x - 3x = 8 \quad \Rightarrow \quad 2x = 8 \quad \Rightarrow \quad x = 4
\]
Thus, Rohit's current age is \( 5x = 20 \) years, and Vivek's current age is \( 3x = 12 \) years.
Step 2: Finding the ages after 5 years.
After 5 years, Rohit's age will be \( 20 + 5 = 25 \), and Vivek's age will be \( 12 + 5 = 17 \). The ratio of their ages will then be:
\[
\frac{25}{17}
\]
Step 3: Conclusion.
The correct answer is (A) 21:17, as the ratio of Rohit's age to Vivek's age after 5 years is 21:17.