Market researchers sent an opinion survey to every twenty-fifth person who ordered a subscription to a weekly news magazine. The researchers were MOST clearly employing a technique known as:

random sampling

Explanation:

Random sampling -

As can be determined from the word, the sampling which is done on the random fashion, is random sampling.

It is the method, by which some samples are distributed to certain random people or object from a group of people or object, is referred to as random sampling.

It is also known as probability sampling.

Hence, from the given scenario of the question,

The correct term is random sampling.