He is using Parallelism, don't let the quiz fool you.
C. Necessary is the answer
Answer:
1. A precious light stones consisting of light bluish green variety of beryl.
2. Artificial pond.
3. Breathing apparatus used by divers.
4. water, saliva,tears, rain.
Each column is called a group. The elements in each group have the same number of electrons in the outer orbital. Those outer electrons are also called valence electrons.