Final answer:
Incompetence is generally the most challenging ground to prove just cause for employment termination due to its subjective nature and the need for well-documented performance standards and improvement opportunities.
Step-by-step explanation:
Of the options provided, incompetence is often considered the most difficult ground for an employer to prove just cause for termination. This is generally because it can be subjective and subject to interpretation.
Factors that might make incompetence harder to substantiate include variability in job performance standards, lack of clear documentation, and the need for the employer to have provided reasonable support and time for the employee to improve. Contrastingly, misconduct, insubordination, and poor performance typically involve more objective measures and are hence easier to document and justify as reasons for dismissal.