What type of sampling is conducted on a one-time basis?

A grab sample is a type of sampling collected on a one-time basis, making it the correct choice here. This method involves taking a single sample of material or a specific volume of water, soil, or another medium at one point in time without any further integration or mixing when compared to other sampling methods. Grab samples are particularly useful when looking for specific conditions or contaminants at a distinct moment, such as measuring acute pollution incidents or momentary changes in water quality.

In contrast, random samples encompass multiple points or instances to ensure that the sample represents a broader population, while composite samples mix together multiple samples taken over time or from different locations to create a representative sample. Continuous sampling, on the other hand, involves ongoing collection over a time frame to track changes and trends rather than capturing a singular moment. Each of these alternatives serves different analytical purposes, highlighting the unique application of grab sampling in situations where a snapshot in time is necessary.