"Mas" is a conjunction that means "but" or "however." It is used to introduce a contrast or exception to what has been said previously.
"A" is a preposition that means "to" or "at." It is used to indicate a destination, location, or direction.
"Of" is a preposition that means "of" or "from." It is used to indicate possession, origin, or material composition.
In the context of the sentence, "mas a" is used to introduce a contrast between the two options. The speaker is saying that something is not true, but rather another option is true.
Therefore, the correct option is "mas a."