In linear programming, the objective function is the function that we aim to either maximize or minimize based on the given constraints.
- Option (A) \( x \geq 10 \) represents a constraint, not an objective function.
- Option (B) \( y \geq 0 \) represents a constraint, not an objective function.
- Option (C) \( z = 7x + 3y \) is a linear equation, which represents an objective function to either maximize or minimize.
Thus, the correct answer is: (C) \( z = 7x + 3y \).