Correct Answer: 60

Solution : Given: The average of D ₁ and D ₂ is 40.
The sum of D ₁ and D ₂ = 40 × 2 = 80 ---------------(1)
The average of D ₂ and D ₃ is 45.
The sum of D ₂ and D ₃ = 45 × 2 = 90 ----------------(2)
The average of D ₁ and D ₃ is 55.
The sum of D ₁ and D ₃ = 55 × 2 = 110 ------------(3)
Adding the equation (1), (2) and (3), we get,
D ₁ + D ₂ + D ₂ + D ₃ + D ₁ + D ₃ = 80 + 90 + 110
⇒ 2 (D ₁ + D ₂ + D ₃ ) = 280
⇒ D ₁ + D ₂ + D ₃ = 140
⇒ 80 + D ₃ = 140
$\therefore$ D ₃ = 60
Hence, the correct answer is 60.