How does this compare to what happened when the Bantu peoples moved south of the equator?

The Nubians didn't really speak the same language as the Egyptians: the Nubians spoke Nubian languages while the Egyptians spoke an Afroasiatic language. They wrote in a variation of a Coptic language, which was used in Egypt.

We could consider this answer:

d. It is different: The Nubians retained Egyptian cultural influences and continued using hieroglyphics as their written language.

but they didn't use hieroglyphs, but a variation of a Coptic script.

The correct answer is: a. It is different: The Nubians retained many Egyptian cultural influences but developed their own written language.