The author rejects both main theories (practical use and social bonding) as complete explanations. The key point is that language became complex even when those needs don’t fully justify it, so humans must have an inherent drive to create and expand language.
Thus, B is correct.
Here is the quick elimination of it.
A. Language originated primarily as a tool for hunting and defense, though social bonding played a secondary role.
This contradicts the author’s main point. The author argues that language cannot be explained mainly by practical or social functions. WRONG.
B. Humans are innately driven to create complex language, beyond purely practical or social purposes.
Correct. The passage concludes that humans are intrinsically motivated to develop and experiment with language, beyond utility or social cohesion. RIGHT.
C. Linguists have overemphasized the importance of small groups in explaining language diversity.
No support for this. The passage does not claim overemphasis by linguists. WRONG.
D. Social bonding explains the stylistic variation observed in early human languages.
Opposite of the argument. The author says social bonding alone cannot explain variation. WRONG.
E. The complexity of language is proportional to the size and needs of the community.
Directly contradicted. Complexity appears even in small groups, so no such relationship is established. WRONG.