Correct Answer - Option 4 : 5%

Ratio of Principal to amount = 4 : 5 ----(i)

Ratio of principal to amount after three years = 5 : 7 ----(ii)

From (i) and (ii), we get

Principal = 20 units, Amount after certain time = 25 units,

Amount after another 3 years = 28 units

⇒ SI = 28 - 25 = 3 unit

As we know,

SI = Prt/100

⇒ 3 = (20 × r × 3)/100

∴ r = 5%