The correct answer is: C. Product of moment of inertia is zero.
The moment of inertia of a body about an axis is a measure of the body’s resistance to angular acceleration about that axis. The principal axes of inertia of a body are the three mutually perpendicular axes about which the body’s moment of inertia is the same.
The product of the moments of inertia of a body about two perpendicular axes is equal to the square of the distance between the axes. Therefore, the product of the moments of inertia of a body about its principal axes is zero.
Option A is incorrect because the sum of the moments of inertia of a body about any two perpendicular axes is equal to the moment of inertia of the body about an axis through its center of mass.
Option B is incorrect because the difference of the moments of inertia of a body about any two perpendicular axes is equal to the product of the moments of inertia of the body about those axes.
Option D is incorrect because the product of the moments of inertia of a body about its principal axes is zero.