Let me begin by saying that I have no clue. My boxing spectating is limited to multi-sports games, and even then it’s not much. That said, I always led to believe that the term “bantam” in English was to do with ghosts/spirits and such. For some reason, I think the word has Thai origins as well, so here’s my thoughts on your choices.
Bantamweight is one of the lower weights, so I could refer to a measurement of rice (like a bag or something). With my association of the word with spirits, perhaps a squid is also possible, but that feels like a bit of a stretch. One could see how a cockfighting term could make its way into the popular vernacular for boxing, but I have no clue if that’s the case. Personally, I be shocked if it’s a reference to a rodent.
Based off my brainstorm and limited knowledge, I’d go with A. Thai rice.
Hope this helps in some way, sorry for my lack of knowledge here.