To solve the problem, we need to determine the rate of simple interest per annum, given that a sum amounts to ₹6,600 in 2 years and ₹7,200 in 4 years.

1. First, let's recall the formula for calculating the simple interest (SI):
   \(\text{SI} = \frac{P \times R \times T}{100}\)where P is the principal, R is the rate of interest per annum, and T is the time in years.
2. We are given two conditions:
   • The amount after 2 years is ₹6,600.
   • The amount after 4 years is ₹7,200
3. Calculate the simple interest earned during the period from the 2nd year to the 4th year:
   • The interest earned in 4 years minus the interest earned in 2 years is: 
     \(\text{SI}_{4 \ \text{years}} - \text{SI}_{2 \ \text{years}} = 7200 - 6600 = 600\)
4. Since the interest over the additional 2 years is ₹600, we can equate it to the simple interest formula for the additional 2 years, i.e., \(\text{SI} = \frac{P \times R \times 2}{100} = 600.\)
5. As P does not change, let's find the total principal:
   • The amount at the end of 2 years is the principal plus the interest for those 2 years: 
     \(6600 = P + \frac{P \times R \times 2}{100}\)
   • And the amount at the end of 4 years: 
     \(7200 = P + \frac{P \times R \times 4}{100}\)
6. By calculating for simple interest earned in 2 years from step 4:
   • The difference between the principal only depends on the interest from step 3: 
     \(\frac{P \times R \times 2}{100} = 600\)
   • Solving for the rate \(R\), use: 
     \(R = \frac{600 \times 100}{P \times 2}\).
7. Hence, solving:
   • Using \( P = \frac{6600}{1 + \frac{R \times 2}{100}} \) which were found equal earlier.
   • From \( \frac{6600}{1 + \frac{R \times 2}{100}} = 6600 - 600 \), and considering:
     • Upon solving the rate:
       \(\frac{600}{2} = \frac{P \times R}{100}\)
     • Simplifying for \(R\), yields \(R = 5\)%.
8. The rate of interest is thus verified as 5% per annum.

Hence, the correct answer is 5%.