The unit of heredity is:

Options

A. chromosome
B. DNA and RNA molecules
C. nucleus
D. gene

Answer

To answer the question "The unit of heredity is:", we need to analyze the options provided:

1. **Chromosomes**: While chromosomes are structures that contain genes, they are not the basic unit of heredity. They are made up of many genes.

2. **DNA and RNA molecules**: These are the molecules that make up genes. However, they are not considered the unit of heredity themselves.

3. **Nucleus**: The nucleus is the cell organelle that contains chromosomes. It is not the unit of heredity.

4. **Gene**: A gene is the fundamental unit of heredity. It is a segment of DNA that contains the instructions for building proteins, which determine traits in an organism.

Based on this analysis, the correct answer is:

**D. Gene**

### Step-by-Step Solution:
1. **Identify the options**: The options given are chromosomes, DNA and RNA molecules, nucleus, and gene.
2. **Evaluate each option**:
- **Chromosomes**: Structures that carry genes but are not the basic unit of heredity.
- **DNA and RNA molecules**: Building blocks of genes but not the unit of heredity.
- **Nucleus**: Contains chromosomes but is not the unit of heredity.
- **Gene**: The basic unit of heredity that carries genetic information.
3. **Select the correct answer**: Based on the evaluations, the answer is gene.