Which of "the following employee groups is most likely to "be excluded by the nlrb from participating in organizing activities and being a member of the bargaining unit?
a) employees in multiple facilities within a single employer.
b) employees covered by multiple employers.
c) employees with certain supervisory duties.
d) employees who have been on strike for economic reasons for less than one year and who have been replaced by other employees.

C) Employees with certain supervisory duties

Explanation:

The NLRB organization consists of:  

1) The Board – have 5 members and their staff

2) The General Counsel- final and independent authority which is under the Board and has to do investigation of charges and issuance of compliance.  

3) The Regional Offices- are located in large cities and are supervised by the General Counsel

NLRB Authority is about the enterprises those affect on the commerce by their operations. There could be included to the commerce: “trade, traffic, transportation, or communication within the District of Columbia or any Territory of the United States; or between any State or Territory and any other State, Territory, or the District of Columbia; or between two points in the same State, but through any other State, Territory, the District of Columbia, or a foreign country”

Supervisors are actually excluded by the NLRB from participating in organizing activities and being a member of the bargaining unit because, he/she could have some interest in favor of employer so there might be inappropriate situations like: to cause another employee to be hired, rewarded, disciplined