Explanation:
In the Big-M method, artificial variables are introduced to convert the problem into a standard form linear programming problem. The objective is to minimize the sum of the artificial variables, which are added to the evaluation row.

When solving the problem using the Big-M method, if the objective function row shows optimality but one or more artificial variables are still in the basis, it indicates an infeasible solution. This means that the original problem does not have a feasible solution.

Here is a detailed explanation of why this is the case:

The Big-M Method:
1. Introduce artificial variables: In the Big-M method, artificial variables are introduced to convert the problem into a standard form linear programming problem. These variables are used to make the constraints feasible.
2. Initial tableau: The initial tableau is formed with the original decision variables, artificial variables, and the evaluation row.
3. Optimality test: The optimality test is performed by checking if all the coefficients in the evaluation row are non-negative.
4. Iterations: Iterations are performed by applying the simplex method to find the optimal solution. The artificial variables are eliminated from the basis by replacing them with the original decision variables.
5. Termination: The iterations continue until the optimality test is satisfied, i.e., all coefficients in the evaluation row are non-negative.

Explanation of the options:
a) Optimal solution: An optimal solution is achieved when the optimality test is satisfied, and all artificial variables are eliminated from the basis. However, in this case, one or more artificial variables are still in the basis, indicating that the solution is not optimal.
b) Pseudooptimal solution: A pseudooptimal solution is a solution that appears to be optimal but is not truly optimal. In this case, the solution is not even pseudooptimal because the artificial variables are still in the basis.
c) Degenerate solution: A degenerate solution occurs when there are multiple optimal solutions with the same objective function value. However, in this case, the solution is not degenerate because the artificial variables are still in the basis.
d) Infeasible solution: An infeasible solution is a solution where the original problem does not have a feasible solution. In this case, when the objective function row shows optimality but one or more artificial variables are still in the basis, it indicates an infeasible solution.

Therefore, the correct answer is option 'D' - Infeasible solution.