Concept:The word "dispose" requires a fixed preposition to form the phrasal verb meaning "to get rid of something".
Explanation:"Dispose of" is the correct phrase in standard English.
It means to throw away, remove, or deal with unwanted things.
For example: "You should dispose of empty containers properly."
The sentence states that cleanliness means removing useless items, so "of" fits perfectly.
The other options do not form correct standard expressions with "dispose".
"Dispose away" is not acceptable usage.
"Dispose with" is rarely used and does not mean "get rid of".
"Dispose off" is a common spelling confusion but is grammatically wrong in formal writing.
Hence, the completed sentence becomes: "An act of cleanliness is to dispose of useless items."
Answer:B. of