Explanation
Correct answer: A.
Choice A is the best answer. The convention being tested is the use of verb forms within a sentence. A main clause requires a finite (tensed) verb to perform the action of the subject. In this case, the subject is "several advantages," and the present perfect tense verb "have positioned" supplies the finite verb to indicate what has made acoustic waves a promising alternative to electrical waves.
Why the other choices are wrong
Choice B
Choice B is incorrect because it results in an ungrammatical sentence. The nonfinite participle "positioning" doesn’t supply the main clause with a finite verb.
Choice C
Choice C is incorrect because it results in an ungrammatical sentence. The prepositional phrase "by positioning" doesn’t supply the main clause with a finite verb.
Choice D
Choice D is incorrect because it results in an ungrammatical sentence. The nonfinite participle "having positioned" doesn’t supply the main clause with a finite verb.