Step 1: Understanding the Question:
An ascribed trait is given by birth. An acquired trait is something a person takes up later by choice. We must find the acquired trait for a Gond tribe member.
Step 2: Check option (1):
Asserting a tribal identity is a choice, but the NTA key does not mark it as the acquired trait here.
Step 3: Check option (2):
Staying in the place of origin is not treated as an acquired trait.
Step 4: Check option (3):
Adopting the caste principle of purity and pollution is a practice taken up from another social system. This is the acquired trait in the NTA key.
Step 5: Check option (4):
Continuing to use one's own language is an inherited trait, not an acquired one.
Final Answer:
The acquired trait is adopting the purity and pollution principle, which is option 3 (as per the NTA final answer key). \[ \boxed{\text{Adopting the purity and pollution principle}} \]