Explanation:

- First, let's break down the given information. The simple interest (SI) for 2 years is Rs. 4200.

- The difference between the compound interest (CI) and simple interest for the same period, at the same rate, is Rs. 420.

- This means the compound interest for 2 years is Rs. 4200 + Rs. 420 = Rs. 4620.

Now, let's find the compound interest for 3 years:

- The formula for compound interest is A = P(1 + r/n)^(nt), but here, we can use the additional interest year concept.

Continuing from Rs. 4620 for 2 years:

- CI for the 3rd year will include interest on this 4620.

Evaluating all options, you get the accurate compound interest amount as the calculations adjust for interest accrual:

- Option 3: Rs. 7644