Semiconductors
Semiconductors are materials that have properties between conductors and insulators. They have the ability to conduct electricity under certain conditions, but not as easily as conductors like copper or aluminum. Semiconductors are widely used in electronic devices such as transistors, diodes, and integrated circuits.

Properties of Semiconductors
To understand why arsenic is not a semiconductor, it is important to understand the properties of semiconductors. Semiconductors have a few key properties:

1. Band Gap: Semiconductors have a small energy gap between their valence band (where electrons are bound to atoms) and their conduction band (where electrons can move freely). This energy gap determines the ability of the material to conduct electricity.

2. Conductivity: The conductivity of a semiconductor can be altered by adding impurities or by applying an external voltage. This property allows semiconductors to be used as switches or amplifiers.

3. Doping: Doping is the process of intentionally adding impurities to a semiconductor to alter its electrical properties. This is done by adding atoms of a different element into the crystal lattice structure of the semiconductor.

Explanation of the Answer
From the given options, the correct answer is option 'C' - Arsenic. Arsenic is a metalloid element, but it is not considered a semiconductor. This is because arsenic has a high conductivity and behaves more like a metal than a semiconductor.

Although arsenic has some semiconductor-like properties, such as a small energy gap between its valence and conduction bands, its high conductivity makes it unsuitable for many semiconductor applications. Arsenic is often used as a dopant in semiconductors to alter their electrical properties, but it is not used as a standalone semiconductor material.

Summary
In summary, arsenic is not a semiconductor because it has a high conductivity and behaves more like a metal. While it has some semiconductor-like properties, it is typically used as a dopant in semiconductors rather than as a standalone semiconductor material. Silicon, germanium, and selenium, on the other hand, are all commonly used semiconductors in electronic devices.