Step 1: Recall what froth floatation is built for.
This process separates ore particles from waste rock by making use of surface wetting behaviour, and it is specifically suited to sulphide ores, whose surface does not get wetted by water easily once treated with collector oils.
Step 2: Check each option for a sulphide.
$ZnCO_3$ is a carbonate ore, $ZnO$ is an oxide ore, $Na_2S$ is a soluble salt rather than an insoluble ore mineral, none of these are sulphides. $ZnS$, zinc blende, is the sulphide ore in the list.
Step 3: Confirm why sulphide ores specifically suit this method.
In froth floatation, air is bubbled through a water suspension of the crushed ore mixed with pine oil. Sulphide particles cling to the oil and get carried up with the air bubbles into a froth that is skimmed off, while the earthy gangue particles, being water loving, sink and stay behind. This mechanism depends on the sulphide surface chemistry, which only $ZnS$ has here.
Step 4: Conclude.
$ZnS$ is the ore concentrated by froth floatation. \[ \boxed{ZnS} \]