Chandragupta Maurya is known to have conquered Magadha primarily through a combination of military strategy, alliances, and exploiting enemy weaknesses. However, his conquest is often attributed to the approach of gradually weakening the enemy.
The most fitting response from your options would be:
by attacking their borders and slowly weakening the outer reaches of the city first.
This reflects his strategy of systematically undermining Magadha's power before launching a full-scale attack.