In the 18th Mercer Quality of Life Study, option 2, Vienna, has been identified as one of the best habitable cities. It is important to note that this is based on the specific study conducted by Mercer, a global consulting firm.

Option 1, Munich, option 3, Zurich, and option 4, Vancouver, are not mentioned in the study as the best habitable cities. However, it is worth mentioning that these cities have also been recognized for their quality of life in various other studies and rankings.

Vienna, the capital of Austria, is known for its high standard of living, excellent infrastructure, and cultural offerings. The city is often praised for its safety, healthcare system, and educational opportunities.

The Mercer Quality of Life Study evaluates cities on various factors such as political stability, crime rates, healthcare, education, recreation, and environmental conditions. It aims to provide insight into the quality of life in different cities around the world.

In conclusion, while Vienna is recognized as one of the best habitable cities according to the 18th Mercer Quality of Life Study, it is important to consider other rankings and studies for a complete understanding of the best cities to live in.