The correct answer is option 'D' - Unskilled fourth class. This classification of labor does not exist and is not commonly used in the categorization of labor.

Classification of Labor:
1. Skilled First Class:
Skilled first-class labor refers to workers who possess specialized skills and expertise in their respective fields. These workers have received extensive training, education, and experience, enabling them to perform complex tasks and handle advanced techniques. Skilled first-class labor is highly valued and often requires a higher level of qualification than other categories.

2. Skilled Second Class:
Skilled second-class labor includes workers who have acquired a certain level of skills and expertise but may not possess the same level of proficiency as skilled first-class labor. They are competent in performing moderately complex tasks and are considered to possess intermediate skills in their respective fields.

3. Unskilled:
Unskilled labor refers to workers who do not possess any specialized skills or formal training. These workers usually perform manual tasks that require minimal training or qualifications. They often engage in repetitive or physically demanding work that does not require a high level of expertise.

Explanation:
The categorization of labor into skilled first class, skilled second class, and unskilled is a common classification used in various industries, including construction, manufacturing, and service sectors. These categories help in determining the skill level and qualifications required for different job roles and tasks.

However, the option 'Unskilled fourth class' does not fit into this classification system. It is not a recognized category used to classify labor. It is possible that this option was included as a distractor or mistake in the question.

In summary, the classification of labor typically includes skilled first class, skilled second class, and unskilled categories. The option 'Unskilled fourth class' is not a valid classification and does not exist in the categorization of labor.