The sentence describes a specific incident where a Darvesh was present near a dry well by chance.
The correct phrase to use here is "happened to be" which means "was present there by coincidence" or "by chance." This fits the narrative tone of a story being told about an event that occurred unexpectedly.
Option (a) "wasn't" changes the meaning entirely, implying the Darvesh was not there.
Option (c) "discovered in" is incorrect because the Darvesh was not discovered inside something but merely passing by.
Option (d) "found to" is incomplete and ungrammatical in this context because it would need to be followed by a verb form like "be" to make sense.
Thus, "happened to be" is the most accurate and contextually correct choice.