In the following passage, there are blanks, each of which has been denoted by a letter. For each blank, five options are given. Choose the most appropriate word from the options that fits the blank appropriately.

The controlled use of fire represents one of humanity's most ______ (A) technological breakthroughs. Anthropologists believe our ancestors first ______ (B) fire naturally caused by lightning strikes or volcanic activity. This accidental discovery eventually led to the ______ (C) of techniques for creating and maintaining flames.

Early humans likely noticed how wildfires ______ (D) their environment, both frightening and fascinating them. The first intentional use of fire for warmth and protection marked a ______ (E) moment in human evolution. Over millennia, our ancestors developed increasingly sophisticated methods to ______ (F) this powerful element, fundamentally changing the course of civilization.

Which of the following words will fit in the given blank (B).

The correct answer is harnessed.

To determine the most appropriate word for blank (B), let's analyze the context of the sentence and the meanings of the given options:
The sentence reads: "Anthropologists believe our ancestors first ______ (B) fire naturally caused by lightning strikes or volcanic activity."

Let's evaluate the options:
1. harnessed: This means to control and make use of a natural resource or force. In the context of early human history, before humans knew how to create fire from scratch, they had to capture, control, and utilize (harness) naturally occurring fires (such as those started by lightning). This fits the context of a technological breakthrough perfectly.
2. extinguished: This means to put out or quench a fire. While early humans might have put out fires, it does not fit the passage's theme of utilizing fire as a major technological advancement that led to the development of techniques for maintaining it.
3. squandered: This means to waste or throw away resource foolishly. This has a negative connotation and does not fit the historical and evolutionary context of utilizing fire.
4. circumvented: This means to find a way around or avoid an obstacle. One does not "circumvent" fire in a way that leads to its controlled technological use.

Therefore, harnessed is the most logical and contextually appropriate word for blank (B).