In a parliamentary republic, a often serves as both head of state and head of government.
a. prime minister
b. king
c. dictator
d. monarch

In a parliamentary republic, a prime minister serves as both head of state and head of government. So the correct option that fits the blank space in the given question is option "a". In a parliamentary form of government, every parliamentarian needs to be elected by the people of the country. Among those elected the prime minister will get selected from the party that is holdiing the majority of seats in the parliament. After a fixed tenure, the procedure has to be repeated and in that case the same prime minister may not get elected or selected. If the opposition from the first tenure wins the second election, then the prime minister will be selected from the opposition party.  

Idon’t know wish i can