Step 1: Eliminate the other rulers:
Chandragupta Vikramaditya was a later Gupta king, Kanishka a Kushana king, and Harshvardhan a 7th-century king -- none received Seleucus's ambassador.
Step 2: Confirm by date:
Megasthenes's mission followed the 303 BCE treaty between Seleucus and Chandragupta Maurya, fixing the answer to the Mauryan founder.
Final Answer:
\[ \boxed{\text{Chandragupta Maurya}} \]