The underlying principle for the NLRB's determination of an appropriate unit is that only employees who have can be appropriately grouped in that unit. a the same number of years of total work experience b the same educational background c similar cultural and regional backgrounds d similar wages, hours, and working conditions

d. similar wages, hours, and working conditions

Explanation:

The National Labor Relations Board (NLRB) is a federal agency founded by Congress in 1935 to administer the National Labor Relations Act (NLRA).

The NLRB determines if the bargaining unit proposed by the labor organization is appropriate by following a framework for organizing.

Their principle is that they group employees of the same work hours and conditions and employees with close or similar wages and salaries in the similar units.

They determine the bargaining unit, after hearings, voter eligibility, and decide which employees should be included in the vote.