💡 Detailed Explanation
The correct choice is 'たばかり' (just finished doing). The grammar pattern '[Verb Past Form] + ばかり' indicates that very little time has passed since the action was completed. Sentence translation: 'Since I have just eaten lunch a moment ago, my stomach is still full.' 'たところ' is also used for immediate completion, but 'たばかり' emphasizes the subjective feeling of recency much better here, and it connects naturally with 'なので' (whereas 'たところなので' is grammatically possible but less idiomatic in this standard N4 context). 'るばかり' and 'るところ' are dictionary-form connections indicating future or progressive actions, which contradict 'さっき' (a moment ago).