Correct answer: A. Treaty and marriage alliance
Explanation: The direct answer is A, treaty and marriage alliance. Chandragupta Maurya came into conflict with Seleucus Nicator, a successor of Alexander in the eastern territories. The conflict ended in an agreement: Seleucus ceded certain territories to Chandragupta, and diplomatic relations were established; traditional accounts also describe a marriage alliance. Megasthenes was sent to the Mauryan court as an ambassador, showing the continuing relationship. Option A is correct because the connection is a treaty, territorial settlement and diplomatic or marriage alliance. Option B, Sangam assembly, belongs to the literary and cultural history of ancient south India, not this Mauryan-Seleucid event. Option C, Gupta conquest, is wrong because the Gupta dynasty came later and was unrelated to Chandragupta Maurya’s agreement with Seleucus. Option D, Buddhist council, refers to religious assemblies and does not describe this diplomatic event. Memory cue: Chandragupta–Seleucus = treaty, territory and diplomacy.