<p>Paul Ecke, flower grower and hybridizer, (became) (known as) “Mr. Poinsetta” after developing new varieties of the flower and (by pioneering) it (as a living symbol) of Christmas. Why is (by pioneering) the correct answer?</p>
<p>IIRC, this is a parallel structure error. Since the sentence already says “after…”, you don’t need the by. Should be “after developing…and pioneering”.</p>
<p>should not be after developing… pioneering by, just pioneering.</p>