Which planet do you think could be colonized, and what technological advancements would it require?
Mars could be colonised with a few improvements on today's technology.
Other than the Earth the only other planet in our solar system which could be colonised is Mars. Mercury and Venue are too hot and the outer planets are gas giants. Some moons could possibly be colonised which include our Moon and Jupiter's moon Europa and Saturn's moon Titan.
One issue is landing safely on Mars. We have sent many unmanned mission to Mars, but quite a few failed to survive landing.
One of the first steps towards establishing a colony on Mars would be to send cargo to supply the colony. This would include food, vehicles and components for buildings. We can do this with existing technology. There is water on Mars in the form of ice near the poles which could be utilised.
Technologies such as 3D printing and inflatable habitats are currently being tested on the International Space Station.
The major technology improvements we need are in terms of getting the colonists to Mars. The journey time is currently several years. During this time the colonists would need supplies and protection from radiation. If better propulsion systems are developed the journey time can be reduced.
Space Agencies including NASA are researching the technologies required to establish a colony on Mars. The ISS is playing an important role in this research. Given enough funding it is feasible that we can establish a colony on Mars in perhaps 20 years.