Concept:Simple interest equals compound interest for the same principal and period.Given: P=6,40,000, R=4.15%, T=3 years 9 months =3.75 years.We need the compound interest rate r for 2 years such that CI=SI.Explanation:Step 1: Calculate simple interest.SI=100P×R×T​=1006,40,000×4.15×3.75​=₹99,600Step 2: Write compound interest for 2 years.CI=P(1+100r​)2−P=P[(1+100r​)2−1]Step 3: Set CI=SI.6,40,000[(1+100r​)2−1]=99,600(1+100r​)2−1=6,40,00099,600​=0.1556(1+100r​)2=1.15561+100r​=1.1556​=1.075100r​=0.075r=7.5%Answer:Rate of compound interest = 7.5%Correct option: C. 7.5%