Respuesta :
Institution in colonial Mexico was, in all probability, the Roman Catholic Church
The answer which best completes the sentence is: the most powerful Institution in colonial Mexico was, in all probability, the Roman Catholic Church.
First of all, it is important to highlight that usually, a comma cannot be placed between a subject and a verb and in this option, the commas separate an additional comment which does not provide relevant information to the meaning of the sentence, so that it could be removed.