Final answer:
The correct word to complete the sentence is 'Genteel', meaning polite and refined, fitting the context of the young ladies' observed manners.
Step-by-step explanation:
The word that best completes the sentence 'The young ladies' manners were noticed by their host; he wondered where they were from.' is c) Genteel. This word describes people who are polite, refined, or respectable, often in an affected or ostentatious way. It fits the context of the sentence, which implies that the ladies had notable manners that caught their host's attention, prompting him to consider their upbringing or origins.
The other options do not fit as well: 'Heterogeneous' means diverse or varied, 'Congenital' relates to traits or conditions present from birth, 'Gentile' pertains to someone who is not Jewish, and 'Engendered' means caused or given rise to.