A profit-maximizing firm in a competitive market that is producing on a production curve where the marginal product of labor is diminishing also has

A. a downward-sloping labor demand curve.

Explanation:

The labor demand curve is plotted with the quantity of labor demanded vs the real wages paid to labor. In a firm that is producing in a market with a diminishing marginal product of labor, the demand curve will be downward sloping to reflect that the more labor that a company has, the less it pays them.

This is because the extra labor is bringing in less additional revenue and so will need to be paid accordingly to reflect that as more labor is hired, the output decreases.  

decentralization can the employees to get more involved in the business decision making

the third answer is correct.