The airline industry is cutting its lowest discount fares more widely, more substantially, and earlier this year than it normally does at the end of the summer, a time during which travel usually decreases and the industry uses some lower fares for the attraction of passengers.

(A) during which travel usually decreases and the industry uses some lower fares for the attraction of- wrong!
the airline industry uses lower fares with an intention to attract the passengers-- when there is an intention involved "to+verbs"are preferred to communicate a better meaning!

(B) during which travel usually decreases and therefore the industry will use some lower fares for the attraction of-wrong!
the two verb tenses decreases and will use are not parallel to each other! and plus we are talking about something that happens always so we must use simple present tense!!

(C) in which travel usually decreases and in which the industry therefore uses some lower fares attracting- wrong!
in which cannot logically refer to time- eliminate C

(D) when travel usually decreases and the industry uses some lower fares to attract- Correct!
"travel decreases" is parallel with "industry uses"

(E) when travel usually decreases and therefore the industry will use lower fares for the attraction of- wrong!
the two verb tenses decreases and will use are not parallel to each other! and plus we are talking about something that happens always so we must use simple present tense!!
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Hi there!
I don't understand how did you strike off A and B on because of absent comma.
In my opinion, it is during.When we say a fixed timing as we are here mentioning "a time", it is legit to use when instead of during.
During presents the period.

What you say!

Hi! I agree with you...which when used as a relative pronoun modifier requires a comma but in this case it doesn't

Posted from my mobile device
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One reason that D is the correct answer is the use of the infinitive "to attract' in comparison to the less idiomatic noun form for 'the attraction of' and the gerund 'attracting" in other choices.
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