First, because it's easier to track for me, I will refer to A as "Acme", B as Bob, and N as "National News Co. (NNC)" for my own sanity.
To address your questions:
Would it be legal for news agency C to show a snippet of this data?
Yes, assuming that NNC took no steps to encourage Bob's theft of the data on the Product X. Given the product... or the related data... was in a confidential state, Acme likely has a non-disclosure agreement with Bob, stating that Bob will not discuss company secrets with people not working with Acme unless Acme expressly tells him to discuss and how much info is too much. As Acme has a contract with Bob they can exercise their claims against him for breach including suit for damages and termination from employment. In the event NNC did influence Bob to steal the X data, NNC may be criminally liable for conspiracy crimes related to any criminal actions of Bob's (likely theft... espionage if these are government secrets... they use Non-disclosure agreements in much the same way.).
Would a small amount qualify for fair use? (as the content is copyright of company A)
Technically no... but technically only. Fair Use is not an exemption giving cart blanche to abuse copyright claims. NNC are using copyrighted data from Acme related to Product X without authorization from Acme. Acme may sue them. Fair Use, however is an "Affirmative Defense" to Copyright Infringement and can be invoked by NNC's legal defense to their use of the copyrighted material. Since they only used a small portion of the X Data available to the public, are likely not competing with Acme, are using the information to inform the public and/or present valid critique of Acme, it stands that it's a good chance they would prevail in a Fair Use defense. They would even point out they are sourcing the data from a crime and as a "Balanced, Trusted, Fair, Non-Fake News Agency in America" has a duty to report on the facts of the crime and allegations of levied by Big-Acme against "whistleblowers" like Bob. NNC likely isn't competing in the same market as Acme (NNC is in the business of reporting news and information to the public. Acme is in the business... of... supplying cartoon coyotes with hunting equipment... sure, let's go with that, so NNC publishing the data is not impacting Acme's own competitive income (that is... the data might be embarrassing for Acme and cause market share loss in favor of Ajax* but it's not because NNC is selling Earthquake Pills that are actually effective on Road Runners.).
Again, Bob is the problem for Acme, not NNC for reporting on what Bob might have done.
