The magazine High Times has a website that once asked visitors whether recreational marijuana use should be legal. This is an example of:

Voluntary response sampling.

Explanation:

As it's name suggests, a voluntary response sample is a sample made up of volunteers, making it biased in the process given that they may or not have strong opinions of the subject at hand. Using the example from the exercise, we can notice that visitors of the website can vote in their poll. As it is free and for any visitor, anyone could enter and vote according to their convictions. Therefore, it is a voluntary response, making it a voluntary response sampling.