If they asked you via phone or questionnaire about if you are applying to other internships, what you should tell them?
The truth?
I thought telling them that you are in current talks with other companies would hurt you.
Why? I guess it might with some, but it might make you appear more competitive with others.
Regardless, lying to a future employer is never the smart way to go. Never mind the ethical issues.
If a company is going to reject you because you are talking to other companies, then you don’t want an internship there. They are either looking for a cultist obsessed with their culture or a reject that they can scoop up cheap, and both are situations ripe for abuse.
It would be ok (if true) to say something like, “Yes, but this position interests me the most because. _". Or "Yes, but I think this position would give me the best experience in .” That will show that you’ve done some research into their company / job posting.
If you’re just applying then it’s up to you. However if you have gotten interviews for other positions it is to your benefit to let them know. This usually helps speed things up. The company I am interning for gave me an offer the day after my final round interview and gave me 5 days to accept when they heard I had 2 pending offers. They were also very generous salary wise. Good luck!