Step 1: Understanding the sentence structure.
For the sentence "Bo doctorni thani thangkha", identify the postposition. Postpositions follow the noun or pronoun they modify, unlike English prepositions which precede them.
Step 2: Analyzing the options.
- (1) Bo: This is the noun, not a postposition.
- (2) Doctorni: This is also a noun, not a postposition.
- (3) Thani: This word functions as part of the sentence, not a postposition.
- (4) -ni: This is the correct postposition, indicating a relational function.
Step 3: Conclusion.
The correct answer is -ni.