You can be compelled to testify, unless doing so would incriminate you. I imagine the law has standards for what sort of information qualifies. There’s always an argument to be made, however. Revealing a source wouldn’t seem to tend to do so, but you never know.
The standard is that YOU get to decide what qualifies. If they want to force you they have exactly one option. Give you full immunity from prosecution.
A child can see 100 ways the government could prosecute the FOX reporter for reporting what they have hidden from us. All that has to happen is that the reporter has to say “i refuse to answer to the grounds that it might tend to incriminate me”.
The judge can then lock her up for contempt, and she can sit and let corporate lawyers start emergency lawsuits and writs to federal courts for her immediate release. She shouldnt say a word. She did a service to gun owners here.
It varies from state to state since there is no Federal shield law. Generally, the state must show clear and compelling evidence regarding damage or potential damage to a case to force a reporter to testify.