Q: Can I use It’s A Trap! if someone uses Zap A Card to take one of my Keepers into their hand?

A: Quick answer: Yes.

Obviously, the more generic Belay That! (counter Action) would work, but the question here is about whether Zapping would trigger the Trap. The card It’s A Trap! is intended to counter Keeper “stealing” in all general senses to include more than just the specific Action Steal A Keeper. It was originally conceived to counter Keepers with special stealing abilities, like The Captain or The Scientist in Star Fluxx, but it also works if someone is invoking Crime Happens (AKA Plunder) to steal one of your Keepers.

So, since Zap A Card essentially lets someone steal one of your Keepers, we would answer yes: you can use It’s A Trap in response to someone Zapping one of your Keepers into their hand. Of course, if they don’t have any Keepers in play themselves, you won’t get anything back, but you will still squander their Zap A Card, and prevent your Keeper from being taken.

Note that You Can’t Take This Guy From Me (Firefly Fluxx) has the same trigger, and can be invoked by all the same situations. Although there is no Zap A Card in Firefly Fluxx, there is a Plunder card, and Zap A Card is available as a promo, so it could be added to any deck.