my pick for D

a) them has no proper antecedent
b)wrong meaning, to me it is not measures that are widely distributed
c) here control widely distributed
e)again it-control widely distributed

B says that the measures to control the borer is widely distributed. - Incorrect.
C and E state that the control is widely distributed. - Incorrect.
A - them has no referent

D is right.

Although the lesser cornstalk borer is widely distributed, control of them is necessary only in the South.

(A) the lesser cornstalk borer is widely distributed, control of them is: them here is not correct
(B) widely distributed, measures to control the lesser cornstalk borer are : here measures to control modifies widely distributed and this is completely meaningless
(C) widely distributed, lesser cornstalk borer control is : lesser cornstalk borer control is not correct
(D) the lesser cornstalk borer is widely distributed, measures to control it are : Here this is correct
(E) it is widely distributed, control of the lesser cornstalk borer is : what does it refer to. it stands alone without any mention and so no clarity.

IMO it is D.

Please mention the correct answer

Let's analyze this one... In A "them" does not seem to agree in number with its antecedant "cornstalk borer". B and C have although without a clause so they can not be right. E has problem of "it" refering to an unknown antecedant. So my pick is D which has a clause following although and correct pronoun usage.

Although the lesser cornstalk borer is widely distributed, control of them is necessary only in the South.

(A) the lesser cornstalk borer is widely distributed, control of them is
"the lesser cornstalk borer" is one entire thing. "The bore" is singular. Them is plural. This sentence is wrong.

(B) widely distributed, measures to control the lesser cornstalk borer are
Although widely distributed, is incorrectly modifying measures and not "the borer".

(C) widely distributed, lesser cornstalk borer control is
"the borer" control is necessary..? This is wrong. should be "measures to control the borer are necessary".

(D) the lesser cornstalk borer is widely distributed, measures to control it are
"it"="the borer".
Although widely distributed, measures to control it(the borer) are necessary. Correct answer.
(E) it is widely distributed, control of the lesser cornstalk borer is

Although it (the borer) is widely distributed, control of it (the borer) is necessary...
We can reduce it further:
Although it (the borer) is widely distributed, control is necessary...

Not the best answer.

D for me too.

+1 for D