(A) Both (A) and (R) are true and (R) is the correct explanation of (A).
Placer deposits in sands of valley floors contain minerals not corroded by water, such as gold, silver, tin, and platinum, which correctly explains why they occur as alluvial deposits.
Source: Minerals and Energy Resources, Mode of Occurrence of Minerals
The textbook explicitly states both facts together: certain minerals occur as alluvial/placer deposits because they are not corroded by water. Since R directly and correctly explains A, option (A) is the right choice. Students often confuse (A) and (B) — choose (B) only when both statements are true but R does not logically explain A.