(For) any mayor of a vast area , an (understanding of) current issues in all districts (is) not only (desirous) but also vital .
For me i see that d that no problem i know that it is a case of parallelism what come after (not only) must comes also after (but also) . ( Desirous) is adj and ( vital ) is also is adj so it is correct parallel structure .
I need to know why D is wrong.
Where did you get this question?
Its a real CB question. The problem is that desirous is not a word. it should be desirable.
http://dictionary.reference.com/browse/desirous
That’s weird. It’s in the dictionary. 
Looking at the examples on that dictionary page, however, it seems that its meaning is not quite “desirable.”
Ex. She is desirous of…
Means “She desires…” not “She is desirable.”
“Desirous” is definitely a valid word.
The error in the original question is that “desirous” should be replaced with “desirable.”
yeah maybe i checked on a poor dictionary 
Thank you all for your help < 3
I stand corrected on the validity of desirous as a word and agree that its not the right word in context
You can think of it as “desirous” being subjective and “desirable” being objective. So you can be desirable to your parents, but desirous of chocolate.