Answer
(a) Increase
(b) option (I)
Work Step by Step
(a) We know that the moment of inertia depends on the mass and radius of the earth. When the dust and small particles add up with the increase in radius, then the moment of inertia increases.
(b) We know that the best explanation is option (I) -- that is, the dust adds mass to the Earth and increases its radius sightly.
