To identify the odd one out among the given options, we need to look for a country that is different or unique compared to the others. Let's analyze each option:
A. San Marino - San Marino is a small independent state located within the borders of Italy. It is the oldest surviving sovereign state and constitutional republic in the world.
B. El Salvador - El Salvador is a country in Central America. It is the smallest and most densely populated country in the region.
C. Lesotho - Lesotho is a landlocked country located in southern Africa. It is known as the "Kingdom in the Sky" because of its high altitude and mountainous terrain.
D. Vatican City - Vatican City is an independent city-state enclaved within Rome, Italy. It is the smallest internationally recognized independent state in the world and serves as the headquarters of the Roman Catholic Church.
The odd one out among these options is B. El Salvador. While the other options are all independent states, El Salvador is a country but not an independent state. It does not have the same level of sovereignty and political independence as the other options. Therefore, the correct answer is B.