Answer: Reliable
Explanation: A self monitoring scale is usually adopted in measuring an individual's ability or willingness to adjust on their behavior, character or general perception by others. Hence, the self monitoring scale is used to measure a stable personality trait.
However, in the question above, the scale outputs a result of 59 on Friday and the same person was given a mark of 28 four days later. The difference between the two result reflects the unreliability in the output generated and cannot be considered as being a stable personality trait output.