The correct option is A thanked
The words 'I ______ Rajni' describes an action that happened in the past.
Options A and C: 'Thanked' is in the simple past tense. It describes an action completed in the past. It's the most suitable option.
'Had thanked' is in the past perfect tense. It describes an action that happened before another commenced. Here, the second action took place first 'for what she had done for me'. Hence this tense is suitable for the second action, and not the one that happens later 'I thanked Rajni'.
Hence, option A is correct and C is incorrect.
Option B: 'Thanks' is a noun and does not express an action.
Option D: 'Was thanking' is in the past continuous tense. But the action here isn't continuous; it's a completed action.
Hence options B and D are incorrect.