Considered to be the most unforgiving course in the world, cyclists must train especially hard to meet the challenge.(A) Considered to be the most unforgiving course in the world, cyclists must train especially hard to meet the challenge.
The portion before the comma is a modifier incorrectly modifying "cyclists". What is "considered to be the most unforgiving course in the world" is not mentioned in the sentence. Additionally, "considered X" is the correct idiomatic usage, not "considered to be". Eliminate.(B) Considered as the most unforgiving course in the world, the challenge is be met only by cyclists training especially hard.
Same error as (A). Additionally, "considered X" is the correct idiomatic usage, not "considered as". Eliminate.(C) Cyclists must train especially hard to meet the challenge of the course considered more unforgiving than any other course in the world.
Correct answer. No dangling modifier and no new errors introduced. (D) The most unforgiving course in the world, the challenge for cyclists is to train especially hard for it.
Again, a dangling modifier. What is "the most unforgiving course in the world" is not mentioned. Consequently, "it" has a missing antecedent. Eliminate.(E) Meeting the challenge requires cyclists to train especially hard for the most unforgiving course in the world.
Meaning change. It appears to imply that the "challenge" is something other than "the most unforgiving course in the world". Eliminate.Hope this helps.