Final answer:
Ponciano Arriaga's speech is a primary source as it is the original document from the 1856-1857 Constitutional Convention for which he was a writer.
Step-by-step explanation:
The speech given by Ponciano Arriaga at Mexico’s 1856-1857 Constitutional Convention is a primary source. Since it is the original speech that was delivered by one of the writers of the Mexican constitution, it represents an original document that has not been subsequently interpreted or analyzed, which is characteristic of primary sources. In contrast, a secondary source would include analysis, interpretation, or evaluation of the primary source material.