The technician reminded them (a)/ to have a thoroughly clearing (b)/ of the machine after each use.(c)/ No error (d)

Answer

2

Explanation

to have a thorough cleaning will replace to have a thoroughly cleaning because -
thorough (Adj.): complete with regard to every de tail
thoroughly (Adv.) : completely, very much
Look at the examples given below:
Planners need a thorough understanding of the subject. He searched the house thoroughly.