Answer:
You cannot challenge an arrangement chosen by the then-living owner of the CD. He/she had the right to choose a beneficiary for that account. The CD is not a probate asset and is not part of his/her estate. For example, I recently arranged for my accounts at one bank to be payable on death to one of my relatives. No one has the right to challenge my choice.
If you think the account owner was not capable of making that decision at the time then it is your responsibility to bring a court action at your own expense and provide enough evidence to convince a judge that you have a legitimate claim.