Why are some lakes salty?
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Answer: Because mineral-rich water enters a closed basin and evaporates, leaving salts.
Because seawater trapped in the basin stays and concentrates as it dries. — Wrong. Trapped seawater can explain a few coastal basins cut off from the sea. Most salty lakes, however, lie inland; their salts arrive dissolved in inflowing water and are concentrated by evaporation with no outlet.
Because mineral-rich water enters a closed basin and evaporates, leaving salts. ✓ — Correct. A salt lake usually sits in a basin with no outlet to the sea. Rivers bring dissolved minerals in; evaporation removes water but leaves the minerals, so salts build up over time. The Dead Sea is about 34% salt, while the ocean is about 3.5%.
Because salt beds inside the basin dissolve and raise the water's salinity. — Wrong. Salt beds can add salt where they exist, so this is not absurd. But most salt lakes get their main salt load from dissolved minerals carried in by rivers, and the lack of an outlet lets evaporation concentrate those minerals.