Tuberculosis, a major cause of death in most of the third world countries is an especially dangerous disease because it spreads through air and requires a complex therapeutic regimen for curing it.
Use of 'especially' is correct in this case. 'especially' is used to draw attention to tuberculosis and point that it is much more dangerous than other diseases.
So we are left with options A, B, C.
(A) an especially dangerous disease because it spreads through air and requires a complex therapeutic regimen for curing it.
Use of curing is wrong. Breaks parallelism. Could have been 'regimen to cure it'.
C) an especially dangerous disease because it spreads through air and requires a complex therapeutic regimen to cure
requires a regimen to cure whom? Does not correctly refer to the disease's cure.
(B) an especially dangerous disease spreading through air and requiring a complex therapeutic regimen for curing it
Correct answer. Fulfills parallelism.