The complexity and variety of organic molecules is due to ________.

• A. the high melting points of organic compounds
• B. the lack of isomerism in organic molecules
• C. the inability of $C$ to form multiple bonds
• D. the presence of different functional groups

Answer: (D)

Verified step by step guidance

1. Understand that the complexity and variety of organic molecules arise from the structural diversity of carbon-containing compounds.
2. Recognize that carbon atoms can form four covalent bonds, allowing for a wide range of bonding patterns including chains, branches, and rings.
3. Identify that the presence of different functional groups (such as hydroxyl, carbonyl, amino, etc.) attached to carbon skeletons introduces varied chemical properties and reactivities.
4. Note that isomerism (structural, geometric, and stereoisomerism) further increases the diversity of organic molecules by allowing the same molecular formula to have different structures.
5. Conclude that the key reason for the complexity and variety of organic molecules is the presence of different functional groups combined with carbon's bonding versatility.