The correct option is B Autocracy
Option B, Autocracy is the correct word for the phrase "Government by a single person". Option A 'Monarchy' is a form of government with monarch (king or queen) at the head. Option C 'Plutocracy' is a system of government where the richest people rule the country . Option D 'Aristocracy' is a form of government where people who hold high social rank rile the country. Thus, options A, C and D are incorrect word for the phrase and the correct answer is Option B, Autocracy.