Which one of the following ores is best concentrated by froth flotation method?

First, recall the basic principle of the froth-flotation method. This method separates gangue from ore by taking advantage of the preferential wettability of the ore particles by oil and that of the gangue particles by water. Because sulphide ores tend to be hydrophobic, they attach to the air bubbles, rise with the froth, and thus get collected, while the gangue sinks.

Therefore, ores that are sulphides are best suited for this process. Let us list the given options with their chemical nature:

We have Galena: $$PbS$$, which is a sulphide ore.

Now consider Malachite: $$CuCO_3\cdot Cu(OH)_2$$, a basic carbonate of copper, not a sulphide, so it is not effectively concentrated by froth flotation.

Next, Magnetite: $$Fe_3O_4$$, an oxide ore of iron, also not suitable for froth flotation.

Finally, Siderite: $$FeCO_3$$, a carbonate ore of iron, likewise unsuitable for froth flotation.

So, among all the options, only Galena is a sulphide ore and therefore responds efficiently to the froth-flotation technique.

Hence, the correct answer is Option A.