Because Fiona is not influenced by merit pay or sales commissions and she performs better in a highly structured job, Fiona has

External locus of control

Explanation:

People with external locus of control tend to believe that the outcomes that happen in their life occurred outside of their control. They believed that their future is something that is pre-determined or destined to be.

These type of people tend to not be influenced by positive incentives. It's hard for them to bring themselves to work harder in order to get better result. This is why these type of people perform better in highly structure job. They prefer being told what to do rather than taking initiatives.