BibTeX:
@incollection{Various,
  author = {Various},
  title = {The Letter of Certainty},
  booktitle = {Baha'i Stories for Children},
  url = {https://bahaistoriesforchildren.blogspot.com/2017/03/the-letter-of-certainty.html},
}

APA:
Various. The Letter of Certainty. In *Baha'i Stories for Children*.

MLA:
Various. "The Letter of Certainty." *Baha'i Stories for Children*, n.d..

Chicago:
Various. "The Letter of Certainty." In *Baha'i Stories for Children*, n.d..