Chandragupta Maurya, the founder of the Maurya Empire in ancient India, is known to have conquered Magadha through a combination of strategies, including the use of military tactics. Among the options provided, while the most historically accurate method of conquest involved strategic military action, the specific detail about using spies to open city gates stands out as a notable tactic in ancient warfare.
Chandragupta's rise involved not only direct military engagements but also forming alliances and exploiting weaknesses within the Magadhan kingdom, especially against the Nandas who ruled at the time. His campaign likely included gathering intelligence, undermining his opponents, and employing strategic military maneuvers rather than solely relying on large-scale assaults or using elephants as the primary tactic.
Therefore, while he may have relied on various tactics such as military pressure and strategic deception, the most consistent historical narrative suggests a multifaceted approach rather than the singular options listed, with espionage being a plausible method among others.