Which of the following seas has the highest salinity?

The salinity of a sea or salt lake is governed by the balance between how much water enters it (rivers, precipitation) and how much leaves it (evaporation, outflow), combined with whether the basin is connected to the open ocean. A basin connected to the ocean is continually diluted by circulating seawater, while an enclosed (endorheic) basin with little inflow and intense evaporation has no way to flush out dissolved salts, so its salinity keeps rising over time.

Applying this to the four seas named here:

• Dead Sea: a fully enclosed, outlet-less basin fed only by a much-reduced Jordan River, in an extremely hot and arid climate with very high evaporation. Salts have concentrated for millennia with nothing to carry them away, pushing surface salinity to roughly 34% - about eight to ten times the salinity of typical open ocean water.

• Red Sea: connected to the Indian Ocean through the Bab-el-Mandeb Strait. Despite strong evaporation in its arid setting, ongoing exchange with the open ocean keeps its salinity around 4%.

• Mediterranean Sea: connected to the Atlantic Ocean through the Strait of Gibraltar. High evaporation raises its salinity a little above the global ocean average, to about 3.8%, but circulation with the Atlantic limits it there.

• Caspian Sea: the world's largest inland body of water and technically endorheic, but it receives enormous freshwater inflow from major rivers, chiefly the Volga, which dilutes it to an average salinity of only about 1.2%.

Cross-check: at roughly 34% dissolved solids, Dead Sea water is dense enough that a person floats on the surface with almost no effort - a well-documented physical consequence of its extreme salinity that the other three seas, with salinity at or only slightly above the ocean average, do not exhibit.

So among the seas listed, the Dead Sea has by far the highest salinity.

Strictly, the Dead Sea is a landlocked hypersaline lake rather than a marine sea, and among true ocean-connected seas the Red Sea is the most saline (about 4%); but this question's own option list treats the Dead Sea as a "sea" in the everyday/exam general-knowledge sense, under which its far higher salinity of about 34% is the answer.