The correct answer is A. Labour.
Labour is the human effort applied to the production of goods and services. It is a factor of production, along with land, capital, and entrepreneurship. Labour is the most important factor of production, as it is the only factor that can create new value.
Skill is the ability to do something well. It is acquired through education, training, or experience. Skill is a necessary but not sufficient condition for labour. A person with the necessary skills may not be willing to work, or may not be able to find a job.
Experience is the knowledge and understanding that comes from doing something over time. It is a valuable asset, as it can help people to be more productive and efficient. Experience is not a necessary condition for labour, but it can make people more valuable workers.
Service is an activity that is performed for someone else, usually in exchange for money. It is a form of labour, but it is not the same as production. Services are intangible, while goods are tangible. Services are also usually consumed at the same time they are produced, while goods can be stored and consumed later.
In conclusion, the correct answer is A. Labour.