Interviewers want to know if you’re genuinely interested in this position or if it’s just one of your many options. Simply, they want to know if you’re their top choice. Honesty is the best policy. If you’re applying for other jobs, say so. You don’t have to necessarily say where you’re applying unless you have another offer. But they might want to know where in the hiring process you are with other companies. You can also mention that you’re actively looking for offers if your interviewer asks.
Example: “I’ve applied to a couple of other firms, but this role is really the one I’m most excited about right now because…”