BibTeX:
@incollection{AbdulBah1886,
  author = {'Abdu'l-Bahá},
  title = {Three Questions and an Answer Unasked: Vaḥíd Before the Báb},
  booktitle = {A Traveler's Narrative},
  year = {1886},
  publisher = {Cambridge University Press},
  url = {https://www.gutenberg.org/ebooks/19300},
}

APA:
'Abdu'l-Bahá. (1886). Three Questions and an Answer Unasked: Vaḥíd Before the Báb. In *A Traveler's Narrative*. Cambridge University Press.

MLA:
'Abdu'l-Bahá. "Three Questions and an Answer Unasked: Vaḥíd Before the Báb." *A Traveler's Narrative*, Cambridge University Press, 1886.

Chicago:
'Abdu'l-Bahá. "Three Questions and an Answer Unasked: Vaḥíd Before the Báb." In *A Traveler's Narrative*, Cambridge University Press, 1886.