Regarding the maintenance of the EIS, what must be checked on a monthly basis?

The correct choice emphasizes the importance of regularly monitoring sample filters in the Emission Inspection System (EIS). Monthly checks of sample filters ensure that they are functioning properly and are not clogged or damaged, which could lead to inaccurate readings during emissions testing.

This routine maintenance step is crucial because effective sample filtration is directly linked to the overall performance of emission testing equipment. Ensuring that sample filters are in good condition helps maintain compliance with environmental regulations and supports reliable measurements of vehicle emissions.

While the other options may seem relevant, they either imply that all filters are disposable without highlighting the importance of monitoring or confuse the intent of the question about maintenance requirements. Thus, emphasizing the importance of checking sample filters on a monthly basis is the most accurate representation of maintenance for the EIS.