Final answer:
Freud would have suggested that an excessive fixation is most likely to contribute to regression, where individuals revert to earlier behaviors when stressed.
Step-by-step explanation:
According to Sigmund Freud, an excessive fixation is most likely to contribute to various psychological issues due to unsuccessful engagement or disengagement at certain psychosexual stages during one's development.
Among the options presented, the concept that aligns best with fixation at a particular stage is regression, where an individual reverts to behaviors characteristic of an earlier developmental stage.
For example, an individual with an oral fixation might regress to thumb-sucking or overeating as an adult when faced with stress.