Which country has the 100 percent urban population?

The Vatican is 100% Urban.

Here is a list of countries by the urban percentage (above 80%). Note, there were some major countries missing from the source, so this list should not be considered all-inclusive.

Guadeloupe: 100%

Gibraltar: 100%

Monaco: 100%

Nauru: 100%

Singapore: 100%

Bermuda: 100%

Puerto Rico: 97%

Belgium: 97%

Martinique: 96%

Kuwait: 96%

Guam: 94%

Northern Mariana Islands: 94%

Virgin Islands: 94%

Western Sahara: 94%

Uruguay: 93%

Iceland: 93%

Luxembourg: 92%

Israel: 92%

Andorra: 92%

Malta: 92%

Qatar: 92%

Australia: 92%

Reunion: 91%

Bahrain: 90%

American Samoa: 90%

Argentina: 90%

United Kingdom: 89%

San Marino: 89%

Bahamas, The: 89%

Saudi Arabia: 88%

Venezuela: 88%

Germany: 88%

Chile: 87%

Lebanon: 87%

New Zealand: 86%

Libya: 86%

United Arab Emirates: 85%

Denmark: 85%

Gabon: 84%

Djibouti: 84%

Sweden: 83%

Brazil: 83%

United States: 80%

Canada: 80%

Korea, South: 80%

[...]

2009, Mainland China: 47% urban, 53% rural

Those with less than 20% Urban are:

Eritrea: 20%

Cambodia: 19%

Lesotho: 18%

Rwanda: 18%

Burkina Faso: 18%

Solomon Islands: 16%

Ethiopia: 16%

Malawi: 16%

Nepal: 15%

Papua New Guinea: 13%

Uganda: 12%

Burundi: 10%

Bhutan: 9%