line of questioning vs line of inquiry

line of questioning and line of inquiry both are nouns.

Word	Noun	Adjective	Verb	Adverb
line of questioning	Yes	No	No	No
line of inquiry	Yes	No	No	No

As nouns, line of questioning and line of inquiry are synonyms defined as:

line of questioning and line of inquiry: an ordering of questions so as to develop a particular argument

line of questioning (noun)	line of inquiry (noun)
an ordering of questions so as to develop a particular argument	an ordering of questions so as to develop a particular argument