I have already bought it, but am just curious.

HD had 4 different box connectors. Cheap ones for NM, BX, and BX/AC, and an
expensive one for MC.

The electrical clerk recommended the NM, but that didn't seem right. I was
tempted by the cheap BX clamps, but took the MC since I only needed two of
them anyhow.

Would code require the ones labeled MC, or would the BX have been adequate?