Final answer:
A manager commits a halo error in performance appraisal by basing the assessment on a single attribute of the employee's performance.
Step-by-step explanation:
If a manager commits a halo error in performance appraisal, the manager may have done the following: c. Based assessment on a single attribute of employees' performance.
The halo effect refers to the cognitive bias in which a person's overall impression of someone can be influenced by one trait, which tends to overshadow other aspects of performance. This can lead to an inaccurate assessment of an employee's capabilities and contributions. It is crucial in a performance appraisal process to evaluate an individual's work over the entire period and across multiple competencies to mitigate the risk of such biases.