Chandragupta Maurya conquered Magadha by employing strategic military tactics, including forming alliances and using espionage. Most notably, he is said to have utilized spies to weaken Magadha from within, opening the city gates for his forces. Additionally, he engaged in direct conflict and gradually wore down the opposition, combining military strength with clever strategies.
However, the most common view of his conquest specifically emphasizes the role of his spies and infiltration in opening up the defenses of Magadha. Therefore, the most accurate response from the options provided would be:
by sending in his spies to open the city gates to his soldiers.