Final answer:
The correct word to complete the sentence is 'feasible,' indicating that it was not practical for Stanley to clean the garage alone, hence the decision for others to help.
Step-by-step explanation:
The correct word to fill in the blank in the sentence 'Since it wasn't feasible for Stanley to clean the entire garage by himself, my brother and I decided to help him' is feasible. This word means that something is possible, practical, or easily or conveniently done. Given the context, it suggests that it wasn't reasonable to expect Stanley to clean the garage alone; therefore, others providing assistance made the task more manageable.
In the examples provided, instances of people helping each other out in different scenarios are illustrated. These examples reflect the idea that collaboration and shared effort can help to achieve tasks that might be too challenging for an individual to handle by themselves.