Hello!
This is an interesting question! I would approach it as a commercial reasoning question rather than a test of technical knowledge. The most important thing is to choose on client and clearly explain why they interest you, instead of trying to name a few clients or show off specific detail.
I'd start by briefly explaining what the client does and the sector they operate in, and then link this to the type of work that the firm does for them. From there, you can explain what kinds of matters you might be exposed to as a trainee and why you would find that interesting or challenging (based on your past experiences or future career goals).
It might also be helpful to mention a current issue or trend that is affecting that client or sector, and how the firm could support them. You definitely don't need to go into legal technicalities - the interviews / graduate recruitment team are usually looking for you to display commercial awareness and clear motivation.
As long as your answer is specific and genuinely reasoned, it should come across well! Best of luck!