Homer executed a deed to his realty known as Parcel A, which contained the following clause: "To church, for so long as the realty shall be used for charitable purposes, but if not used for charitable purpose, to Youth Center for so long as the realty shall be used as a center for the youth." Church for the elderly and Youth Center were both charitable institutions devoted to the needs of indigent persons. On the day after the deed was executed, Homer's interest in Parcel A is best described as

Select one:

a. A valid right of re-entry.

b. A valid reversion.

c. A valid possibility of reverter.

d. Void under the Rule Against Perpetuities.