A (n) is a way of thinking or a set of ideas about particular subjects .

D

Explanation:

the answer is axiom

the reason some people sacrifice their own life for another person's life could be that they are in love with them and would do anything to keep them safe a protected even if that means risking their life in the process

the pronoun reference replaces a noun, but you have to make sure there's no room for confusion.

in the first sentence "her" could be referring to shely or to the sister. the answer there is "shelly called her sister and told her about the new baby."

in the second sentence "it" could be referring to the roof or the floor. the answer there is "since the roof leaks on the family room floor, we need to repair the roof."