..A… may be defined as an atom or group of atoms joined in a specific manner which is responsible for the characteristic chemical properties of the organic compounds.
Here, A refers to

Options

A. non functional group
B. functional group
C. aromatic group
D. Non – aromatic group

Answer

To solve the question, we need to identify what "A" refers to in the context of organic chemistry, specifically regarding the definition provided.

### Step-by-Step Solution:

1. **Understanding the Definition**: The question states that "A" is defined as an atom or group of atoms joined in a specific manner responsible for the characteristic chemical properties of organic compounds. This indicates that "A" plays a crucial role in determining how a compound behaves chemically.

2. **Identifying Key Terms**: The phrase "characteristic chemical properties" suggests that "A" is related to the functional behavior of organic molecules. In organic chemistry, certain groups of atoms impart specific properties to compounds.

3. **Examples of Functional Groups**: The video mentions various examples such as hydroxyl groups (–OH), halogens (like chlorine), aldehydes, and carboxylic acids. Each of these groups alters the properties of the base hydrocarbon (like butane) to form different classes of compounds (like alcohols, haloalkanes, etc.).

4. **Defining Functional Groups**: The groups mentioned are known as functional groups. A functional group is a specific group of atoms within a molecule that is responsible for the characteristic chemical reactions of that molecule.

5. **Conclusion**: Based on the definition and examples provided, we can conclude that "A" refers to "functional groups." These groups are essential in organic chemistry as they dictate the chemical behavior and properties of organic compounds.

### Final Answer:
A refers to **functional groups**.