1. Carbohydrate = energy- rich organic compound (c)

Carbohydrates are one of the three biomolecules that biotic organisms use as energy sources (the other two are proteins and lipids).

2. Carbon = element that is part of most organic compounds (d)

Organic compounds are made by biological processes. Examples of organic compounds are lipids, proteins, and carbohydrates. Most organic compounds contain carbon atoms. 

3. Water = inorganic compound (a)

Inorganic compounds are are not organic ("in" = not). These are substances that are made during geological processes (such as rocks, minerals, and salts).

4. Oxygen = element found in water (b)

Water is made of two hydrogen atoms and one oxygen atom (H2O).