Which variations are incorrect and why?
Which are/is the best?
Thank you

a.
As to the water shortage, I'm not exactly sure what you are worried about.
b. I'm not exactly sure what you are worried about
as to the water shortage.
c.
As for the water shortage, I'm not exactly sure what you are worried about.
d. I'm not exactly sure what you are worried about
as for the water shortage.
e.
Regarding the water shortage, I'm not exactly sure what you are worried about.
f. I'm not exactly sure what you are worried about
regarding the water shortage.g.
Concerning the water shortage, I'm not exactly sure what you are worried about.
h. I'm not exactly sure what you are worried about
concerning the water shortage.
i.
With regard to the water shortage, I'm not exactly sure what you are worried about.
j. I'm not exactly sure what you are worried about
with regard to the water shortage.
k.
With regards to the water shortage, I'm not exactly sure what you are worried about.
l. I'm not exactly sure what you are worried about
with regards to the water shortage.
m.
In regards to the water shortage, I'm not exactly sure what you are worried about.
n. I'm not exactly sure what you are worried about
in regards to the water shortage.
o.
In regard to the water shortage, I'm not exactly sure what you are worried about.
p. I'm not exactly sure what you are worried about
in regard to the water shortage.