Explanation

The Arabian Sea has high evaporation (hot, arid surroundings) and receives little freshwater — few major rivers drain into it and rainfall is scanty — so salts concentrate. The Bay of Bengal, by contrast, receives enormous freshwater influx from the Ganga–Brahmaputra system and peninsular rivers plus heavy monsoon rain, diluting its salinity. Shallowness (B) and enclosed land (C) are not the drivers, and the Arabian Sea receives less, not more, rainfall (D is wrong). Hence A.