The correct response is:
This would lead to a shift to the left in the market supply curve for labor.
When workers are incentivized to provide their services, such as through improved working conditions, it typically leads to an increase in the willingness of workers to supply their labor. This would actually shift the supply curve to the right, not to the left, as more workers would be attracted to the labor market due to better incentives. However, since none of the options correctly reflect this, the statement about a shift of the supply curve does not accurately match the expected outcome of improving working conditions.
To summarize: Improved working conditions usually increase labor supply, causing a rightward shift, rather than a leftward shift.