I believe the answer is that 'only a person's good deeds remain with him or her in the afterlife'. It is important to be a good person and do good, and everything else is fleeting, it will disappear. But you will be awarded for your good deeds.
C. The platypus, which is a mammal, has a bill like a duck and feet like an otter.
"which is a mammal" is an adjective clause. An adjective clause is used to describe a noun and usually begins with one of these words: that, when, where, who, whom, whose, which, and why. In this sentence, the adjective clause begins with "which" and describes the platypus, which is a noun.
The correct answer would be poetry
Examples (both in chapter 1): Infinitely small sound and dry rain
