No it is not. It is an adverb phrase since it lacks a subordinating conjunction.
Adverb clauses must be preceded or started by a subordinating conjunction such as after, if, because or although. For a sentence to qualify as an adverb clause it must include in its body a subject and a verb.
As a general rule you can use the following formula:
An adverb clause equals a subordinate conjunction plus a subject plus a verb, and it should be answering how? when? where? or why?
Answer:
it is to inform readers of how hard the life of a slave was and give readers a glimpse into the life of a slave and he they weren't cared about
Using information and knowledge I already have, I would say your answer would be C. A hidden meaning.