The seawater at the mouth of the mississippi river (where it runs into the gulf of mexico) has than the open water of the pacific ocean.
a higher density
a lower salinity
a lower temperature

The correct option is LOWER SALINITY.
The river Mississippi is a fresh water river which has considerably less salt than the pacific ocean which is very salty. So, at the point where the two met, the quantity of the salt there will be lower compared to the sea water of the pacific ocean.