Which of the following is NOT a filter approved by NIOSH?

The choice indicating M100 as NOT being a filter approved by NIOSH is accurate because NIOSH has specific classifications for respirator filters that are widely recognized in the industry, including P100, N100, and R100.

P100 and N100 filters are designed for protection against particulate matter, with P100 offering a high level of efficiency (99.97% filtration) against solid and liquid aerosols, while N100 filters provide the same high efficiency but are not resistant to oil-based aerosols. R100 filters are also approved by NIOSH but are intended for use in environments where the contaminants are not oil-based and offer a lower level of protection compared to P100 and N100.

The designation M100 does not exist in the NIOSH filter classification system, which clearly outlines the categories and specifications for respiratory protection filters. This lack of an M100 designation means that it cannot be considered an approved filter by NIOSH, making it the correct answer to this question.