The South African government takes full ownership of a major mining company to ensure greater control over mineral resources and increase public revenue. This action is best described as:

The question describes a scenario where the South African government takes full ownership of a major mining company. This action is undertaken to gain greater control over mineral resources and increase public revenue.

Let's evaluate the given options:

• A. Privatisation: This refers to the transfer of ownership of a business or service from the public sector (government) to the private sector. This is the opposite of what is described.
• B. Nationalisation: This is the process by which a national government or state takes over the ownership and control of a private company or industry. This aligns perfectly with the description in the question.
• C. Commercialisation: This involves managing an organization or activity in a way that is intended to make a profit, often by adopting business practices. It does not specifically refer to a change in ownership from private to public.
• D. Deregulation: This is the process of removing or reducing government regulations, typically in an economic sector. This is unrelated to the government taking ownership of a company.

Therefore, the action described is best termed nationalisation.

The final answer is $\boxed{\text{B. Nationalisation}}$.