Final Answer:
A user manual is an example of expository writing. Option A is answer.
Step-by-step explanation:
Expository writing aims to convey information, explain a topic, or instruct the reader. Option a, a user manual, perfectly fits this definition. User manuals provide clear and concise instructions on how to operate a device or perform a specific task, making them a quintessential example of expository writing.
Options b and c, a history book and an op-ed article, respectively, may contain elements of expository writing, but they often include narrative or persuasive elements. Option d, an autobiography, is more likely to be categorized as narrative writing, as it typically involves the personal experiences and reflections of the author.
Option: a. A user manual is the answer.