Final answer:
The magnetic moment of an atom arises from both the orbital and spin motions of its electrons. A stationary charged particle does not produce a magnetic field, making the assertion true but the reason false; the correct option is C.
Step-by-step explanation:
The question posed concerns the origin of magnetic moments in atoms, and specifically whether assertion (A) that the magnetic moment of an atom is due to both the orbital motion and spin of every electron is correct, and whether reason (R) that a charged particle at rest produces a magnetic field correctly explains this assertion.
Assertion (A) is true, as the magnetic moment of an atom is indeed due to both the orbital motion and the spin motion of electrons. Each electron in an atom generates a magnetic field through its orbital motion, known as the orbital magnetic moment, and through its intrinsic property called spin, leading to the spin magnetic moment. The combination of these two contributions to the magnetic field determines the overall magnetic moment of the atom.
Reason (R) is false because a charged particle must be in motion to produce a magnetic field. A stationary charged particle does not produce a magnetic field, but rather an electric field. Thus, reason (R) does not correctly explain assertion (A).
The correct option in both parts of the question is answer C: (A) is true but (R) is false.