Which of the following is NOT a type of penetrant?

The selection identifies viscosity as a term that does not denote a type of penetrant. In liquid penetrant testing, penetrants are categorized based on their visible characteristics and functionality. The primary classifications are fluorescent and visible penetrants.

Fluorescent penetrants are designed to emit visible light when exposed to ultraviolet (UV) light, making it easier to detect flaws. Visible penetrants, on the other hand, can be seen under normal lighting conditions and do not require UV light for visibility.

The term "Type II" refers to a specific classification under the American Society for Testing and Materials (ASTM) standards and pertains to a category of penetrants, not a standalone type.

Viscosity, however, describes a physical property of a penetrant, specifically its resistance to flow, rather than a type of penetrant itself. Therefore, viscosity does not fit into the classification of penetrants used in liquid penetrant testing, which is why it's the correct answer to this question. Understanding the different types of penetrants and their properties is crucial for effective application of liquid penetrant testing methods.