If a person inherits a Roth IRA, and the original owner was older than their beginning date, the beneficiary must withdraw all the money and close the account within 10 years, and must take RMDs each year until the account is distributed. None of the proceeds are taxable. If B dies before the inherited IRA is spent out, C inherits it as beneficiary under the same terms.
If a trust is the designated beneficiary, the distributions are still not taxable, but the account must be closed out in 5 years, and RMDs must be taken in the mean time.
What you are really asking is an estate planning issue. For example, if B dies before C and C might be too young to manage their money. You need more sophisticated and personalized estate planning advice.
