The mass of Mars is 6.42 × 1023 kg. Its moon Phobos is 9378 kilometers away from Mars and has a mass of 1.06 × 1016 kg.

Use Newton’s law of gravitation: F = Gm₁m₂/r². Convert the distance to meters: 9378 km = 9.378 × 10⁶ m. Substituting the masses and G = 6.674 × 10⁻¹¹ gives F ≈ 5.16 × 10¹⁵ N, matching choice A.

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