The correct answer is Option B.
In the given question we are to choose the word that means 'a government by one person'.
In the options given all the words end with 'cracy'.
'Cracy' means 'a particular form of government'.
Thus in Option A- ARISTOCRACY- On breaking the word we get two parts - ARISTO and CRACY. 'Aristo' means the nobility or the higher noble class. Therefore, ARISTOCRACY will mean 'a form of government which is ruled by the nobility.
Similarly in Option C and D:
Option C- We get 'DEMO' and 'CRACY', which means a form of government ruled by representatives of the population.
Option D- We get 'BUREAU' and 'CRACY' which means a form of government ruled by state officials rather than representatives of the people.
None of the above are therefore the required answer.
Option B - AUTOCRACY- On breaking the word we get 'AUTO' and 'CRACY'.
AUTO means one person. Therefore, AUTOCRACY means a form of government governed by one person.
Hence Option B is the correct answer.