Hey Elliot,
For this question, one way to approach it is to determine what the passage said about cagA, which states that it causes an inflammatory response. Out of the options available, leukocytes was the most plausible as it had to do with inflammatory/immunity responses. If this didn't seem intuitive to you, not to worry! We cover immunity and all related topics in BioBiochem class #7. Some of the questions right now might require some prior knowledge you have not seen, but we will definitely get you ready for whatever you will need to face!
Hope this helps,
Molly