Imo. E
When training dogs for the Iditarod, providing a nutritious high calorie diet is at least as important as maintaining a regimented exercise and training routine for the musher.
When training dogs for x, ??? Subject, i.e. musher, should be come in place immediately after comma; modifier error. There is no subject for is in second part of the sentence and it has nonsensical meaning.
A. When training dogs for the Iditarod,
providing a nutritious high calorie diet is at least as important as maintaining a regimented exercise and training routine
for the musher. - Incorrect, Same as above reason.
B. When
they train dogs for the Iditarod, providing a nutritious high calorie diet is at least of
equal importance to the musher as maintaining a regimented exercise and training routine. - Idiomatically incorrect. Pronoun agreement error,i.e. they referring to whom? It must be musher which is singular.
C. Mushers training dogs for the Iditarod find that providing a nutritious high calorie diet
to be equally important as maintaining a regimented exercise and training routine. - as important as is idiomatically correct.
D. For mushers training dogs for the Iditarod,
providing a nutritious high calorie diet is at least as important to them as that they maintain a regimented exercise and training routine. - Comparison marker, as ... as needs parallelism agreement. Here, providing and maintain are not parallel to each other. Besides, sentence uses unnecessary pronouns, leading distortion of meaning.
E. For mushers training dogs for the Iditarod, providing a nutritious high calorie diet is at least as important as maintaining a regimented exercise and training routine. - Sounds perfect and no error in the sentence.