Concept:
Fundamental theorem of Linear Programming:
quote
If an optimal solution exists, then it occurs at a corner (vertex) point of the feasible region.
quote
Step 1: Examine each statement.
\[
\begin{aligned}
(A) &:\quad \text{False. Some LPPs have no optimal solution.} \\
(B) &:\quad \text{False. An unbounded feasible region may not have a finite optimum.} \\
(C) &:\quad \text{Not always true; feasibility and objective behaviour must be considered.} \\
(D) &:\quad \text{Fundamental theorem of LPP. Correct.}
\end{aligned}
\]
Step 2: Apply the theorem of linear programming.
Whenever an optimal solution exists,
Optimal value occurs at a corner point
of the feasible region.
\[\begin{aligned}
\boxed{\text{Option (D)}}
\end{aligned}\]
Hence, option \(\mathbf{(D)}\) is correct.