A market researcher is investigating the use of public transport in a metropolitan area. To reflect the demographic profile of the city's commuters, the researcher needs to obtain a sample of 200 commuters consisting of 120 bus users, 60 train users, and 20 tram users.

The researcher stands outside a central transport interchange during the evening rush hour and approaches individuals, asking which mode of transport they primarily use. If their category's target has not yet been met, they are invited to complete the survey. Once the target of 120 bus users is reached, no more bus users are surveyed, and this process continues until all three targets are filled.

Which of the following best describes the sampling method used in this study?