Step 1 (Definition & Governing Law): [½ Mark]
According to the NCERT definition of Linear Programming Problems (LPP), a linear function of the form $Z = ax + by$, where $a$ and $b$ are constants, which has to be maximized or minimized is called the Objective Function.

Step 2 (Step-by-Step Identification): [¼ Mark]
Evaluating the given options based on LPP terminology:

• Feasible function: Incorrect. The "Feasible Region" is the set of all points satisfying the constraints, not the function itself.
• Objective function: Correct. This is the standard term for the optimization target.
• Optimal function: Incorrect. "Optimal Value" refers to the maximum or minimum value obtained, not the function.
• Constraint: Incorrect. Constraints are the linear inequalities or restrictions like $ax + by \leq c$.

Step 3 (Final Concluded Answer): [¼ Mark]
The linear function to be maximized or minimized is the objective function.
Correct Option: (B) an objective function