Good job getting rid of A, B, and C for clear list structure/parallelism violations--there are extraneous appearances of "and" in all three, and there is an absence of the comparison marker "more" in A and B.
"When" is indeed usually followed by a clause, although sometimes this clause can appear in reduced form, without the verb "to be" ("When in Rome...do as the Romans do").
One more point about choice D: the expected idiom, as the previous poster included in his/her preferred option, is "make it easy (for someone) TO
" do something. The thing that is "made easier" in this case is the action of getting in and out . With "when" replacing the proper preposition "to," the meaning of the sentence becomes unclear. What is the "it" that is made easier for the passengers?
Hope this helps.
PS I don't have a problem with the phrase "getting in and out."
JP Park | Manhattan GMAT Instructor | Los Angeles
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