Keep in mind what you call plenum is not what the NEC calls it. A plenum is inside ductwork, wiring is not allowed inside unless necessary to act on air inside.
We call a plenum a plemum as that is the type of wire typically used.
		
		
	 
300.22 Wiring in Ducts Not Used for Air Handling, Fabricated Ducts for Environmental Air, and Other Spaces for Environmental Air (Plenums). The provisions of this section shall apply to the installation and uses of electrical wiring and equipment in ducts used  for dust, loose stock, or vapor removal: ducts specifically fabricated  for environmental air; and other spaces used for environmental air  (plenums).
Informational Note: See Article 424, Part VI, for duct heaters.
(A) Ducts for Dust, Loose Stock, or Vapor Removal. No wiring  systems of any type shall be installed in ducts used to transport dust,  loose stock, or flammable vapors. No wiring system of any type shall be  installed in any duct, or shaft containing only such ducts, used for  vapor removal or for ventilation of commercial-type cooking equipment.
 (B) Ducts Specifically Fabricated  for Environmental Air. Only wiring methods consisting of Type MI cable,  Type MC cable employing a smooth or corrugated impervious metal sheath  without an overall nonmetallic covering, electrical metallic tubing,  flexible metallic tubing, intermediate metal conduit, or rigid metal  conduit without an overall nonmetallic covering shall be installed in ducts specifically  fabricated to transport environmental air. Flexible metal conduit shall  be permitted, in lengths not to exceed 1.2 m (4 ft), to connect  physically adjustable equipment and devices permitted to be in these fabricated ducts.  The connectors used with flexible metal conduit shall effectively close  any openings in the connection. Equipment and devices shall be  permitted within such ducts only if necessary for the  direct action upon, or sensing of, the contained air. Where equipment  or devices are installed and illumination is necessary to facilitate  maintenance and repair, enclosed gasketed-type luminaires shall be  permitted.
 
(C) Other Spaces Used for Environmental Air (Plenums). This section shall apply to spaces not specifically fabricated for environmental air-handling purposes but used for air-handling purposes as a plenum. This section shall
 70-144 not apply to habitable rooms or areas of buildings, the prime purpose of which is not air handling.
Informational Note No. 1: The space over a hung ceiling used for  environmental air-handling purposes is an example of the type of other  space to which this section applies.
 Informational Note No. 2: The phrase “Other  Spaces Used for Environmental Air (Plenum)” as used in this section  correlates with the use of the term “plenum” in NFPA 90A-2009, Standard for the Installation of Air-Conditioning and Ventilating Systems, and other mechanical codes where the plenum is used for return air purposes, as well as some other air-handling spaces.
B would be limited
C is what he referenced for the location of the light