VERITAS PREP OFFICIAL EXPLANATION:
As you scan this problem for actionable Decision Points, recognize that each answer choice includes a modifier describing "Burj Khalifa" (you can tell this by the comma right after "Khalifa," the subject of the sentence).
Choice (A) is correct, although that fact may come as a surprise. "Opened" and "reigning" are each participial (verb-based) modifiers that properly describe the Burj Khalifa, and then after the modifier you're provided with the active verb "stands" so that you have a complete sentence: Dubai's Burj Khalifa stands more than a half mile tall.
It may not seem that the two modifiers are parallel, but recognize that the verb tenses must stand the way that they are: "opened" (past tense) describes an action that already happened (in 2010), while "reigning" is ongoing ("ever since") so it cannot be in the past tense. So the modifiers are each used appropriately.
With the other choices:
By including "which" as essentially a "subject" for "is reigning" (B) violates the timeline. It could say "which has been reigning ever since" but that word "since" begs for any active verbs to be in the present perfect tense. Note that in (A) "reigning" is used only as a modifier; in (B) by following a noun "which" the verb is subject to stricter verb tense logic.
(C) adds "it has been" to break up the parallel modifier structure between the commas. "Opened and reigning" are parallel modifiers, but "Opened and it has been reigning" mixes a modifier and a clause within the same set of commas, creating an error.
(D) fails to complete a full sentence. If you take "which opened..., has been reigning..., and stands" as three parallel items, they're all part of the "which" modifier and therefore there is no actual verb for the subject "Burj Khalifa." Or if you take only "which opened..." as the modifier, then there should not be a comma between the parallel verbs "has been reigning" and "stands." (Note also that this interpretation would create an illogical meaning, as "has been reigning as the tallest building" and "stands a half mile tall" are related - one should really modify the other - but using them as parallel, separate verbs illogically treats them as separate events).
And choice (E) not only breaks parallel structure in the combination modifier ("opened" is a participial modifier and "which reigns" is a relative modifier) but also fails to have an active verb, as "standing" after the modifier does not work. ("The Burj Khalifa standing a half mile tall" is not a complete sentence.)