The correct answer is option 'B': The number of persons usually employed or willing to be employed.

The labour force is a term commonly used in economics and refers to the total number of individuals who are either currently employed or are actively seeking employment. It includes both those who are employed and those who are unemployed but actively looking for work. Let's delve into the details to understand this concept more comprehensively.

1. Definition of labour force:
The labour force is a key economic indicator that helps to measure the number of people who are available to work and contribute to the production of goods and services in an economy. It consists of two categories: employed individuals and unemployed individuals.

2. Employed individuals:
The labour force includes individuals who are currently working and earning income through formal employment, self-employment, or part-time work. Employed individuals are considered part of the labour force because they are actively involved in economic activities and contribute to the overall productivity of the economy.

3. Unemployed individuals:
The labour force also includes individuals who are currently without a job but are actively searching for employment opportunities. These individuals are considered unemployed and are part of the labour force because they are willing and able to work. They may be actively seeking work through job applications, interviews, or other job-seeking activities.

4. Exclusion from the labour force:
Certain groups of people are not considered part of the labour force. These include individuals who are not actively seeking employment, such as full-time students, retirees, and individuals who are unable to work due to disabilities or other reasons. Additionally, individuals who have given up looking for employment due to discouragement are also excluded from the labour force.

5. Importance of measuring the labour force:
Understanding the size and composition of the labour force is crucial for policymakers, economists, and businesses. It helps in determining the overall employment situation, calculating unemployment rates, and formulating appropriate economic policies. It also aids in analyzing the demographic and socioeconomic characteristics of the labour force, such as age, gender, education levels, and employment trends.

In conclusion, the labour force refers to the total number of individuals who are either employed or actively seeking employment. It encompasses both employed individuals and those who are unemployed but actively looking for work. Measuring the labour force provides valuable insights into the employment situation and aids in policymaking and economic analysis.