"I'm not sure if she's telling the truth, but I'll

Solution

The idiom "take it with a grain of salt" means to be skeptical about something and not fully believe it. In this case, the speaker is unsure about the truth of what someone is saying, so they will approach it with skepticism. The other options are unrelated idioms. "pull someone's leg": tease or deceive someone in a playful way "let the cat out of the bag": reveal a secret or something that was meant to be kept private "hit the nail on the head": identify or say something that is exactly right or accurate