Key A unchanged; displayed explanation described an unrelated fluorite-structure question and did not address co-precipitation.
Explanation
U(IV) fluoride (UF₄) is insoluble and is carried down with an insoluble fluoride of a cation of similar size and charge type. Ca²⁺ (≈112 pm, CN 8) is close in size to U⁴⁺ (≈100 pm), and CaF₂ has the fluorite lattice that actinide(IV) compounds adopt (UO₂, ThO₂), so uranium fluorides co-precipitate with CaF₂.
AgF is very soluble, and LiF and MgF₂ have small cations in rock-salt/rutile lattices, so they do not carry uranium. Answer: (a) CaF₂.