D. Purpose or the reason for which something is done or created or for which something exists.
This was at the begining of the age of the industrialisation, factory work was very harsh, workers had to live in a very small and crowded space, many weren't insured and didn't get enough money for working 12 or more hours a day.
“About Mars” is the Adjective Phrase.
Adjective phrase describes “what kind of which ones”
So for example here
“After school, I looked for books about Mars.”
“About Mars” describes the kind of books so that makes it an Adjective phrase.
I hope this helps.
