Typically, the structure "make it easy" or "make it easier" is followed by an infinitive, perhaps an infinitive with the subject of the infinitive in a "for" prepositional phrase.
. . . make it easy for passengers to get in and out . . .
Choice (A) would be correct if it didn't have the "and" at the beginning messing up the parallelism.

What's subtle and funny about (D) is that it almost feels as if there's a missing infinitive . . .
. . . make it easier for passengers to do something? when getting in and out . . .
Because the infinitive is so often used, and so often follows a "for" preposition, having something else makes the sentence feel as if an infinitive should be there somehow.

Also, the construction of (D) is technically illogical--it's a colloquial construction that native speakers use frequently in informal conversation, but it doesn't withstand logical analysis. Here's the correct construction again:
. . . make it easy for passengers to get in and out . . .
What is made easier? The act of getting in and out. The "dummy it" refers to the infinitive. That make sense.
Now, (D).
. . . make it easier for passengers when getting in and out . . .
What is made easier? The "dummy it" appears to refer to the subordinate clause beginning with "when." The "when" clause denotes a time. How is time itself "made easier?" Yes, yes, colloquial, people say stuff of this sort all the time ("she made lunchtime easier," "our end-of-year shopping package makes your holidays easier") It's colloquial and technically illogical. What is meant is that we something easier for someone at that time--we didn't make it easier for the time itself.

All of those are problems with (D).