He ____________ the assignment just now.

• The correct sentence is- 
"He has finished the assignment just now."

• Rules of the formation of this sentence:
- Already, just, yet, recently, lately, ever, just now, still etc ইত্যাদি হচ্ছে present perfect tense indicator বা নির্দেশক।
- অর্থাৎ বাক্যে Already, just, yet, recently, lately, ever, just now, still নির্দেশক শব্দগুলো থাকলে বাক্যটি সাধারণত present perfect tense এ হয়।

• Present perfect tense এর structure: Subject+ has/have +Verb এর past participle + object.

সুতরাং, নিয়মানুযায়ী শূন্যস্থানে সঠিক উত্তর হবে- has finished
Complete sentence: He has finished the assignment just now.