The most useful property of metalloids is their (A) Varying ability to conduct electric current.
(B) Softness and malleability.
(C) Tendency to be unreactive.
(D) Ability to be pulled out into long wires.

(A) Varying ability to conduct electric current.

Explanation:

The most useful property of metalloids is their varying ability to conduct electric current.

Metalloids are elements or substances that shares properties that transcends between those of a true metal and non-metal.

answer is: naturally occurring solid, inorganic compound or element is mineral.

minerals usually have crystalline form, that means than crystal constituents (atoms, molecules or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions.

for example magnetite is a mineral of iron oxide (fe₂o₃).

calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (caco₃).

dolomite is an   carbonate mineral composed of calcium magnesium carbonate (camg(co₃)₂).