Which of the following systems of government allows citizens to hold only one form of political opinion?
a.
a parliamentary republic
b.
a single-party government
c.
a presidential republic
d.
a constitutional monarchy

As far as I am concerned, the following systems of government which allows citizens to hold only one form of political opinion is the option : B - a single-party government. I consider this option to be correct because one of the characteristics and main points of a single-party government is actually people's ability in having only one political opinion. 
Do hope it will help you!
Regards.