As shown in the figure, two thin coplanar circular discs $A$ and $B$ each of mass ' $M$ ' and radius ' r ' are attached to form a rigid body. The moment of inertia of this system about an axis perpendicular to the plane of disc $B$ and passing through its centre is

• A

  $2 \mathrm{Mr}^2$
• B

  $3 \mathrm{Mr}^2$
• C

  $4 \mathrm{Mr}^2$
• Correct answer
  D

  $5 \mathrm{Mr}^2$

Hence, the correct answer is option: 'D'.