The First Union Territory of India to run 100 percent on solar energy is

The answer is Diu. This is a current-affairs item, so the answer depends on the official claims made around 2019-20, and different states and cities have made similar 'first' claims. Diu was widely reported as the first Union Territory to meet its power needs fully from solar, supported by rooftop and ground-mounted plants under the national solar mission.

Why the others are weaker:

• Chandigarh is often called a solar-friendly city, with rooftop solar mandates in its building bylaws, but it is a city-level claim and not the UT-level 100% solar supply headline.
• Puducherry has pushed solar through its own policies and subsidies, but its headline targets were about expansion and not complete solar supply.
• Andaman and Nicobar depends on diesel generation on its islands, so it does not fit a 100% solar claim.

The concept is the National Solar Mission under the Jawaharlal Nehru National Solar Mission (2010), along with the rooftop solar push. This links to the GS-III energy and climate change syllabus and to Mains questions on decentralised renewable energy and island grids.

Trap: aspirants pick Chandigarh because it is the most famous 'solar city'. Check whether the question asks about a city or a UT, and whether it says 100% supply.