Step 1: Eliminate other sessions:
The Karachi session (1931) dealt with fundamental rights, the Lucknow session (1916) produced the Congress-League Pact, and the Calcutta session (1928) had only called for Dominion Status within a year.
Step 2: Confirm:
The explicit 'Purna Swaraj' resolution belongs specifically to the Lahore session of 1929.
Final Answer:
\[ \boxed{\text{In the Lahore session}} \]