Which answer combines the following ideas with an adjective clause while retaining the meaning of the original sentences?
john keats wrote poetry. he lived in the nineteenth century.
john keats wrote poetry in the nineteenth century.
john keats, who wrote poetry, lived in the nineteenth century.
in the nineteenth century, john keats wrote poetry.

The correct answer is: John Keats, who wrote poetry, lived in the nineteenth century. Adjective clauses usually start with pronouns like who, whom, which, or that and adverbs like when, where, and why that give a direct description of the subject of the sentence. The adjective clause in the sentence is "who wrote poetry," which directly describes John Keats. The clause connects with another idea that John Keats was a man who lived in the 19th century. 

seeing as no choices were provided, i'll do my best to answer it from knowledge.

job specialization promotes work ethic to the person doing the job. he/she may be strong in the arms and able to lift things, specializing him/her in a job where you lift and carry.