<p>A makes no sense because it means sadly nostalgic (I believe)
B means having one’s confidence destroyed.
C means stated with conviction. The author is definitely making a stance when he says “The idea… is wrong.” So C is the closest choice.
D is completely contradictory because the author is definitely not undecided.
E either means regretfully explaining what happened or defending by words or arguments. (I believe that the second definition of apologetic is outdated, by the way.) And E has nothing to do with making a stance.</p>