What property makes metalloids unique in comparison to metals and non-metals?

Metalloids are unique due to their ability to conduct electricity under specific conditions, which distinguishes them from both metals and non-metals. This property stems from their intermediate electrical conductivity, which makes them valuable in electronic devices. At room temperature, metalloids may act as insulators, but when exposed to heat, light, or impurities, they can exhibit conductivity similar to metals. This semi-conductive behavior is essential in applications such as semiconductors, which are critical for technology like transistors and diodes.

The other options do not accurately describe metalloids. They do not exist solely in a gaseous state, as they can be solids at room temperature. While some metalloids can exhibit reactivity, they are generally not classified as "highly reactive" like certain metals or non-metals. Finally, metalloids possess some metallic properties, such as luster and malleability, contrasting them with non-metals, which do not exhibit these characteristics.