A secondary source provides interpretation, analysis, or summary of primary sources or events. Based on the options provided:
A. an encyclopedia article about a factory - This is a secondary source because it analyzes and summarizes information about factories, based on various primary sources.
B. a photograph taken inside a factory - This is a primary source as it is an original image capturing a specific moment in time.
C. an account book of factory finances - This is a primary source as it records actual financial transactions of a factory.
D. a journal kept by a factory worker - This is also a primary source since it provides firsthand accounts of the worker's experiences and observations.
Therefore, the correct answer is A. an encyclopedia article about a factory.