Step-by-step explanation:
Try putting all the words into the first blank. Do they sound right or seem to make sense? You're looking for an adjective, which describes something. The only word I could find that describes something else is customary, which means "done according to custom, usual practice or habit." It means it's normal to do that thing.
They give us the words "to use the." That means that they're using something.
Right now the sentence is "it is customary to use the____." Now I think we are looking for a noun, which is a person, place, or thing. Remember that we are using this, so it's probably a thing, not a person or place. Look at every word on the list and their definition to find anything that fits the blank.
The only word I could find that makes sense is stethoscope, meaning "instrument used to listen to sounds within the body." That makes sense, right? It's normal to use this tool. It could also be customary to use the dictionary.
It is customary to use the stethoscope.
It is customary to use the dictionary.
Hope this helped!