The formula for compound interest is:
\[ A = P \left(1 + \frac{r}{100} \right)^t \]
\[ A_1 = 6000 \times 1.15 = 6900 \]
After paying Rs. 1,200, the remaining balance is:
\[ 6900 - 1200 = 5700 \]
\[ A_2 = 5700 \times 1.15 = 6555 \]
After paying Rs. 1,200, the remaining balance is:
\[ 6555 - 1200 = 5355 \]
\[ A_3 = 5355 \times 1.15 = 6158.25 \]
Thus, the total amount Roselin has to pay at the end of the third year is Rs. 6,158.25. The correct answer is (A) 6,158.25.