Step 1: Identify the milestone.
Cochin International Airport (CIAL) in Kerala, India became the world's first fully solar-powered airport by installing a large solar plant on its premises that generates enough electricity to meet all of the airport's energy needs.
Step 2: Eliminate other options.
Vienna Airport, Changi Airport, and Denver International Airport have solar energy initiatives, but none of them operate entirely on solar power.
Step 3: Confirm the answer.
CIAL's achievement earned global recognition as a landmark in sustainable and green aviation infrastructure. \[ \boxed{\text{Cochin International Airport, India}} \]