Final answer:
Providing additional training is an action that can increase employee motivation by enhancing their effort-to-performance expectancy.
Step-by-step explanation:
The action that would primarily increase employee motivation by enhancing their effort-to-performance expectancy is providing additional training.
When employees receive additional training, they become more confident in their abilities and skills, which increases their belief that their efforts will lead to successful performance. This enhances their effort-to-performance expectancy and motivates them to put in more effort.