Explanation
Correct answer: D.
Choice D is the best answer. The convention being tested is subject-modifier placement. This choice makes the noun phrase "the Alaska Centennial Commission" the subject of the sentence and places it immediately after the modifying phrase "wanting…Purchase." In doing so, this choice clearly establishes that the Alaska Centennial Commission—and not another noun in the sentence—wanted to celebrate the 100th anniversary of the Alaska Purchase.
Why the other choices are wrong
Choice A
Choice A is incorrect because it results in a dangling modifier. The placement of the noun phrase "a contest" immediately after the modifying phrase illogically suggests that the contest wanted to celebrate the 100th anniversary of the Alaska Purchase.
Choice B
Choice B is incorrect because it results in a dangling modifier. The placement of the noun phrase "an award of ＄300" immediately after the modifying phrase illogically suggests that the award of ＄300 wanted to celebrate the 100th anniversary of the Alaska Purchase.
Choice C
Choice C is incorrect because it results in a dangling modifier. The placement of the noun phrase "＄300" immediately after the modifying phrase illogically suggests that the ＄300 wanted to celebrate the 100th anniversary of the Alaska Purchase.