Study Design Help

#1
Hi everyone,

Could anyone help me understand what study design was used in this scenario?

"Researchers took a sample of 1000 people from Los Angeles and ask them if they received the flu vaccine for 2019/2020 as well as if they contracted the flu (Y/N) in that winter season. In order to measure the effectiveness of the vaccine, they compare the percentage of individuals who received the vaccination and contracted the flu to the percentage of individuals who did not receive the vaccine and contracted the flu."

Multiple Choice:
Randomized controlled trial
Ecological study
Case Study
Cross-sectional study
Cohort study
Case control study

Thanks!