This whole debate between "tunnel" and "completion" seems like one of those times when we should just say "who cares; what's the real difference anyway?" The tunnel was an engineering masterpiece, as was its completion, and the entire project to achieve both. The completion of the tunnel linked the two places, as did the tunnel itself. There's very little meaningful distinction to be had, so it wouldn't even matter to me if someone argued that it was ambiguous (though I don't actually think it is). None of this bothers me. And every other answer is terrible. We have to make decisions relative to the options -- Sentence Correction is not meant to be done in a vacuum.
If you forced me to decide, I'd say that a participle modifier that is not at the end of a clause will modify what it is next to -- i.e. "the Holland Tunnel," and that the subject, "completion," is the perfectly logical "masterpiece" to which the verb and object refer.
The rule for participle phrases, as I understand and teach it, is that they will normally modify what they are next to, unless they are at the end of a clause and set off by a comma, in which case they cannot modify what they are next to and will instead modify the preceding clause as a whole.
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but comma+doing always work as adverbial modifier.
Note that I do not agree with this whatsoever (though I also find the adverbial/adjectival distinction largely unproductive).