BibTeX:
@incollection{RhyyihKhnum1969,
  author = {Rúhíyyih Khánum},
  title = {The Guardian's Letter to the United Nations},
  booktitle = {The Priceless Pearl},
  year = {1969},
}

APA:
Rúhíyyih Khánum. (1969). The Guardian's Letter to the United Nations. In *The Priceless Pearl*.

MLA:
Rúhíyyih Khánum. "The Guardian's Letter to the United Nations." *The Priceless Pearl*, 1969.

Chicago:
Rúhíyyih Khánum. "The Guardian's Letter to the United Nations." In *The Priceless Pearl*, 1969.