One of the most common responses of nationalist leaders in developing countries where unrest was common and poverty and ethnic tensions were at...

The correct answer to this question is Option A.  None of the other options make sense.

Option B has the nationalist leaders establishing boundaries to separate the ethnic groups.  This is not something that nationalist leaders ever do.  Leaders do not like the idea of breaking up their countries. It reduces their power and definitely reduces their prestige.  It makes them look bad because it looks like they cannot keep their countries together.

Option C is definitely not correct.  No nationalist leader would ask the European colonizers to return. Any leader who did so would surely face a rebellion.  This goes against everything that nationalists want.

Option D is possible, but it is not what happened. In general, leaders wanted to keep power at all costs.  They did not want to risk losing power in an election. This is why so many developing countries ended up with dictators.

This brings us to Option A.  This is the typical pattern for developing countries like those you describe.  The first leaders want to keep power so badly that they turn to military dictatorships.