Step 1: Context.
The Indian National Congress, under the presidency of Jawaharlal Nehru, convened at the Lahore Session of December 1929.
Step 2: Declaration.
During this session, the Congress officially declared Poorna Swaraj Complete Independenc(E) as its goal, rejecting Dominion Status. The resolution was adopted on 31st December 1929.
Step 3: Independence Day.
Following this, 26th January 1930 was celebrated across India as the first “Independence Day,” laying the foundation for choosing the same date for the Republic Day later.
Thus, Nehru declared Poorna Swaraj at the Lahore Session in 1929.
\[
\boxed{C \text{(Lahore, 1929)}}
\]