Final answer:
b) Purlins. Purlins are horizontal beams that are placed perpendicularly to other trusses or beams to support roof decking.
Step-by-step explanation:
The series of beams placed perpendicularly to other trusses or beams to help support roof decking are called purlins.
Purlins are horizontal beams that are installed parallel to the rafters or joists and are used to provide additional support to the roof structure. They help distribute the weight of the roof evenly and prevent sagging or bowing of the roof decking.
For example, in a trussed rafter roof system, purlins are often added between the rafters to provide extra stability and load-bearing capacity.