"Wamarekani" is plural. Shouldn't the correct answer have "Americans" to be consistent?
"Your guests are Americans" is accepted as an alternative translation for this sentence, because "Wamarekani" can be understood as an adjective or a noun.
I consider the sentence of @Gzeebzee quite correct to express "No, your (sg.) guests are Britons/British" due to the fact that there is not such accurate differentiation between English and British in Swahili as some would like to see. For both, the word stem ingereza is used.
Briton, British, Englishman, Englishwoman = Mwingereza (sg.), Waingereza (pl.), used as adjective or as noun for animate beings (m/wa noun class)
English, English language = Kiingereza, lugha ya Kiingereza
Great Britain, England = Uingereza
United Kingdom = Ufalme wa Muungano
This is so inconsistent! Most times it just says you've got it right with a typo. This time it had to be exact.