Single party ‘Baath party’ is allowed to rule, in _______________.
A.Iran
B.Iraq
C.Yemen
D.Syria

Hint: A political party is an organised group of individuals who share the same ideology and contest elections at the local, state or national level. States around the world have different party systems. A one-party system is in which one political party has the right to form the government. A two-party system is in which two major political parties are competing for power. A multi-party system means that there are a large number of parties competing to attain power both at the national and state level.

Complete step by step solution:
Option A: is incorrect. The Islamic Republic Party was the only political party that had been ruling Iran until its dissolution in 1987. In the late 1990s, multiple parties were formed in Iran and thus Iran became a multi-party state.
Option B: is incorrect. The Ba’ath Party was founded during the Ba’ath movement. In 1966, the party split into two factions, out of which one was Iraqi dominated and the other was Syrian dominated. In 1968, a coup was led by the Ba’ath party in Iraq along with the military to consolidate power and oust the President. This is widely known as the 17 July revolution after which the party exercised a stronghold on the government until the American invasion of Iraq in 2003.
Option C: is incorrect. Although the Republic of Yemen is a multi-party state, the politics was dominated by a single party, the General People’s Congress for a very long time. President Ali Abdullah Saleh served from Yemen’s unification in 1990 to his resignation in 2012 following the Yemeni uprising.
Option D: is correct. After the Ba’ath party split into the Iraqi and Syrian factions, they along with President Bashar al-Assad’s family have remained the dominating party in Syria after the Corrective Movement in 1970.

Note: Some examples of one-party systems are the Chinese Communist Party in the People’s Republic of China. Countries like the USA, UK, Canada, Australia roughly follow the two-party system. A multi-party system is prevalent in France, Germany, Brazil, India, New Zealand, etc.