The correct answer is C. planning.

Planning is the primary function of management because it is the process of setting goals and objectives, and developing strategies to achieve them. It involves identifying the resources that will be needed, and allocating them in the most efficient way possible. Planning also involves monitoring progress and making adjustments as needed.

Staffing is the process of hiring and training employees. It involves identifying the skills and qualifications that are needed, and finding the best candidates to fill those positions. Staffing also involves developing and implementing training programs to ensure that employees have the skills and knowledge they need to do their jobs effectively.

Directing is the process of motivating and leading employees to achieve organizational goals. It involves setting clear expectations, providing feedback, and resolving conflicts. Directing also involves creating a positive work environment where employees feel valued and respected.

Decision making is the process of choosing between alternative courses of action. It involves identifying the problem, gathering information, evaluating alternatives, and selecting the best course of action. Decision making also involves implementing the decision and monitoring its results.

All of these functions are important, but planning is the foundation on which all other management functions are built. Without a plan, it is difficult to make effective decisions, staff the organization effectively, or direct employees towards common goals.