Final answer:
The most likely rock to form when magnesium-enriched seawater reacts with limestone in an evaporating lagoon is 2) dolostone, as magnesium ions replace calcium ions in calcite to form dolomite.
Step-by-step explanation:
As magnesium-enriched seawater in an evaporating lagoon infiltrates the subsurface and reacts with limestone, the rock most likely to form would be dolostone. Limestone is rich in calcium carbonate (CaCO3), and when magnesium from seawater reacts with limestone, it leads to the replacement of some of the calcium ions in the calcite of the limestone with magnesium ions, transforming it into dolostone which is primarily composed of the mineral dolomite (CaMg(CO3)2).
Other rocks listed, such as halite (salt), are formed from the evaporation of sodium chloride-rich waters. Conglomerate is a sedimentary rock composed of rounded fragments and would not form from the infiltration of seawater. Chert consists mainly of silica and is not directly related to the reaction between magnesium and limestone. Lastly, diorite is an igneous rock and is not formed through the processes involving seawater and limestone.