1. The problem asks to identify the sampling method used when a university selects 10 floors out of 180 floors across 30 residences and interviews everyone on those floors. 2. The total number of floors is $30 \times 6 = 180$ floors. 3. The university selects entire floors (clusters) and interviews everyone on those floors, rather than selecting individuals randomly from all floors. 4. This method is called **cluster sampling**, where the population is divided into clusters (floors), some clusters are randomly selected, and all individuals in those clusters are surveyed. 5. Other methods like stratified sampling involve selecting individuals from each subgroup, systematic involves selecting every k-th individual, convenience involves selecting easily accessible individuals, and multi-stage involves multiple sampling steps. 6. Therefore, the correct answer is **Cluster sampling**.