While proposing the relationship between language and thinking process, Lev Vygotsky conceptualized language as a/an :

Lev Vygotsky, a prominent psychologist, emphasized the social nature of learning and cognitive development. He proposed that language is a central tool in the development of cognitive functions.  Key Points

• Vygotsky viewed language as a cultural tool that facilitates cognition in multiple ways. He argued that language enables individuals to organize their thoughts, solve problems, and interact with others, playing a key role in the development of higher cognitive functions.
• Language, as per Vygotsky, is essential for social interaction, and through dialogue and communication, individuals can engage in collaborative learning and cognitive development.
• Language acts as a bridge, linking thought and social experience, and allows individuals to internalize cultural knowledge.

Hint

• Form of egocentric speech used to only fulfill basic needs is incorrect because Vygotsky did acknowledge egocentric speech (self-talk in young children), but he did not limit it to only fulfilling basic needs. Instead, he saw it as an important step toward self-regulation and higher-order thinking.
• Hindrance to the development of cognitive abilities is incorrect. Vygotsky did not consider language a hindrance. Instead, he saw it as a facilitator of cognitive development.
• Independent domain which is not related to cognition is incorrect. Vygotsky believed that language is deeply integrated into cognition and is not independent of it. Language and thought are closely linked.

Hence, the correct answer is cultural tool which facilitates cognition in multiple ways.